Podcasts about fei fei

For the first time, the story of space isn't just about astronauts. It's about farmers using satellite data, engineers designing zero-gravity labs, scientists discovering new drugs, and policymakers crafting the rules. The architects of our future in space are building it from the ground up. On the show: Niu Honglin, Steve & Fei Fei

Chunyun, the annual Spring Festival travel rush in China, is the world's largest human migration. Each year, it tests the absolute limits of the country's transportation network. In 2026, however, it will test something new: how data, platforms, and policy can fundamentally reshape the experience of mass mobility itself. / Is boredom good for us (15:25)? On the show: Niu Honglin, Steve & Fei Fei

More protein. Whole milk returns. Ultra-processed foods take a step back. The latest U.S. dietary guidelines have triggered a wave of reactions—some cheering, others confused. Has nutrition science changed its mind, or are we simply rediscovering what was overlooked? On the show: Niu Honglin, Steve & Fei Fei

A rocket retrieved from the ocean and a strategy to deploy over 200,000 satellites across 14 constellations are not isolated headlines. They are signals of a deeper structural shift redefining the global space industry. We explore the rise of China's commercial space sector and examine its tangible, far-reaching implications for our collective future. On the show: Niu Honglin, Steve & Fei Fei

What connects the quantified self movement, a major fishing ban, and a viral“cry-cry horse"? On the surface, these three stories from this past week appear to exist in separate worlds—one in personal technology, another in environmental policy, and the last in internet culture. We trace how they intersect to reveal a surprising, singular theme about modern systems of monitoring, compliance, and emotional response. On the show: Niu Honglin, Steve & Fei Fei

This is the story of a river that shapes a nation. In the final episode of our series, we trace a decade of evolution along the Yangtze, where regions producing half of China's GDP have rewritten their growth model. They have shifted from a focus on scale to a new paradigm of smart, coordinated integration, a transformation that now offers the world a distinct blueprint for building a sustainable future. On the show: Niu Honglin, Steve & Fei Fei

The 2026 consumer goods trade-in program presents a streamlined process for consumers: claim a voucher, place an order, and receive an immediate discount. Behind this user-friendly interface, however, lies a highly complex system. It requires the exact coordination of central funding, local execution, integrated digital platforms, and real-time verification mechanisms. So what's everyone buying? On the show: Niu Honglin, Steve & Fei Fei

How do you revive a great river? In the first episode of our three-part series, we look at the boldest move yet on the Yangtze: a total fishing ban. And it's working. Native fish are returning and the finless porpoise has reappeared in growing numbers, all while hundreds of thousands of fishermen have traded their nets for new lives on land. Join us for a story of ambitious ecological healing and profound human adaptation along one of China's most important waterways. On the show: Steve, Fei Fei & Yushan

Imagine a hospital visit where the longest wait is the elevator ride. Now, picture paying for care with a single glance, or shielding your family with one shared insurance wallet no matter where in the country they live. This is not science fiction. It is the new reality in China's healthcare system. Today, we explore the digital keys to this future and how they are transforming the patient experience from stressful to seamless. On the show: Steve, Fei Fei & Yushan

What if every green purchase earned you tangible rewards? A major new national plan is betting that a points-for-discounts system is the key to unlocking mass green consumption. This is how the initiative could turn your environmental choice into real savings, making sustainability the smartest buy on the shelf. On the show: Steve, Fei Fei & Yushan

Our cities' greatest resource isn't buried underground; it's hidden in plain sight within the very waste we discard. We journey into the world of urban mining to discover how yesterday's forgotten trash is being reclaimed as tomorrow's essential materials and how this process is turning our cityscapes into a new frontier of resource wealth. On the show: Steve, Fei Fei & Yushan

China has officially entered a new era of demographic governance with the unveiling of the draft Childcare Services Law. While only about 8% of children under three are currently enrolled in formal childcare, the central government is now positioning it not as a private luxury but as a "basic public service." The draft introduces a mandatory national qualification system and comprehensive background checks. This marks a significant step toward professionalizing a sector long dominated by less regulated and often costly private providers. On the show: Niu Honglin, Steve & Fei Fei

After ten years of "Do Not Disturb" signs, the most powerful room in the Forbidden City is back in business. Want to see where the Qing emperors actually lived, worked, and… gossiped? The Hall of Mental Cultivation is finally open. / Why do we doom-scroll our partner's ex (14:49)? On the show: Niu Honglin, Steve & Fei Fei

What happens when your closest confidant is an algorithm? As millions form bonds with AI companions, China is responding with pioneering rules for this wild new frontier of digital intimacy. The crackdown on synthetic dependency seeks to build guardrails against emotional manipulation and preempt real-world harm. On the show: Steve, Fei Fei & Yushun.

Pink Himalayan salt sells for ten times the price of ordinary table salt, wrapped in an aura of wellness and luxury. But what are you really buying? Is it a superior mineral source, or are you paying for little more than a pretty color and clever marketing? We investigate the surprising truth behind the pink. / The Soapbox (14:44). On the show: Steve, Fei Fei & Yushun.

As 2026 arrives, China pauses for a three-day national holiday, a dedicated celebration of new beginnings. Join us as we explore this cultural moment where tradition meets modernity, immersing you in the distinctive sounds, cherished flavors, and personal rituals of a nation taking a collective breath before stepping forward into the promise of the year ahead. On the show: Steve, Fei Fei & Yushan

In late 2025, the most resonant piece of self-care advice in China didn't come from a lifestyle guru, but from an internet meme: 爱你老己 (Love yourself). A single, linguistically clever phrase blossomed into a cultural phenomenon. Its power was in repackaging the universal truth of self-love into something that felt intimate, shareable, and distinctly modern. This is the story of how language itself became a premier tool for wellness in the digital age. On the show: Steve, Fei Fei & Yushan

The year 2025 has reached its final chapter, and it's time for all of us to say our goodbyes to the year. To mark this special year-end send-off, we're opening the doors wide for a final "Motivational Happy Place." We've gathered all of our guests, each ready to share a heartfelt, motivational message with our listeners to close out 2025 together. On the show: Steve, Niu Honglin, Yushan, Fei Fei, Laiming & Yushun

Ready to crack the cultural code of 2025? This year's viral words were more than just slang. They were secret signals revealing our collective fears, our defining humor, and the obsessions we couldn't scroll past. We decode the language that truly defined the last twelve months and uncover why these specific terms captured a global moment. On the show: Steve, Fei Fei & Yushun

Ever get the feeling your headphones are listening more than you are? You might be onto something. While nearly 60 million wireless units shipped in China in just the first half of this year, the real story is a design revolution. Headphones are no longer just an accessory; they're being engineered to become a seamless, intelligent part of your daily rhythm. On the show: Steve, Fei Fei & Yushun

China is drafting a law to protect Antarctica's environment, aligning with its international responsibilities under the Antarctic Treaty System and aiming to regulate activities on the continent to promote preservation and scientific cooperation. This proactive step reinforces China's commitment to sustainable governance and global ecological stewardship in one of Earth's most fragile and vital regions. On the show: Steve, Fei Fei & Yushun

It's human nature to want to put a face to a voice, and that instinct is exactly what's fueling the move toward video podcasts. This isn't just a new format; it's deepening the connection between creators and their audience and opening up entirely new ways to tell stories. On the show: Steve, Fei Fei & Yushun

A health management regulation will take effect soon in the City of Hangzhou, east China's Zhejiang Province. How can legal policies help improve people's health? Today we look at how new laws are turning lifestyle into legislation—from your waistline to your daily routine. On the show: Steve, Fei Fei & Yushan

A startling fact: reported rates of youth depression in China range from 4% to 41%. Why the huge gap? New research points to a fundamental flaw in the diagnostic tools themselves. This has major implications for treatment and policy. / The socks scandal: A 10-Minute guide (14:47). on the show: Steve, Fei Fei & Yushan

What if the next leap in artificial intelligence isn't about better language—but better understanding of space?In this episode, a16z General Partner Erik Torenberg moderates a conversation with Fei-Fei Li, cofounder and CEO of World Labs, and a16z General Partner Martin Casado, an early investor in the company. Together, they dive into the concept of world models—AI systems that can understand and reason about the 3D, physical world, not just generate text.Often called the “godmother of AI,” Fei-Fei explains why spatial intelligence is a fundamental and still-missing piece of today's AI—and why she's building an entire company to solve it. Martin shares how he and Fei-Fei aligned on this vision long before it became fashionable, and why it could reshape the future of robotics, creativity, and computational interfaces.From the limits of LLMs to the promise of embodied intelligence, this conversation blends personal stories with deep technical insights—exploring what it really means to build AI that understands the real (and virtual) world.  Follow Fei-Fei Li:https://x.com/drfeifei Follow Martin Casado:https://x.com/martin_casado Stay Updated: If you enjoyed this episode, be sure to like, subscribe, and share with your friends!Find a16z on X: https://x.com/a16zFind a16z on LinkedIn: https://www.linkedin.com/company/a16zListen to the a16z Podcast on Spotify: https://open.spotify.com/show/5bC65RDvs3oxnLyqqvkUYXListen to the a16z Podcast on Apple Podcasts: https://podcasts.apple.com/us/podcast/a16z-podcast/id842818711Follow our host: https://x.com/eriktorenberg  Check out everything a16z is doing with artificial intelligence here, including articles, projects, and more podcasts. Please note that the content here is for informational purposes only; should NOT be taken as legal, business, tax, or investment advice or be used to evaluate any investment or security; and is not directed at any investors or potential investors in any a16z fund. a16z and its affiliates may maintain investments in the companies discussed. For more details please see a16z.com/disclosures. Hosted by Simplecast, an AdsWizz company. See pcm.adswizz.com for information about our collection and use of personal data for advertising.

What if your car could legally say,“I'll take it from here?" China just crossed that threshold, issuing its first permits for conditional self-driving cars on public roads. But what does“conditional”actually mean for drivers—and what tough turns still lie ahead on the path to full autonomy? On the show: Steve, Fei Fei & Yushun.

Forget personal reading goals. This is about a national ambition. Imagine a government that sees its core job as clearing the path between you and a good book. This is the vision now driving China, transforming reading from a quiet hobby into a public project. We explore the reality of a society built for readers. / Motivational Monday (19:29). On the show: Steve, Fei Fei & Yushun.

In a small county in China, a quiet revolution is reshaping a global jewelry industry. In Zhecheng, central China's Henan Province, lab-grown diamonds are produced at a fraction of the traditional cost. This phenomenon is a clear signal of how China's formidable local economies are becoming pivotal drivers of global markets. What does this county-level prowess mean for the future of global manufacturing and consumption patterns? On the show: Steve, Fei Fei & Yushan

The distinctive scrape of skates carving ice now echoes in unexpected corners of China. From the tropical south to the northern heartland, a new generation is embracing hockey's grit and speed, recently celebrated in a historic championship sweep. This is a story of a sport on fast-forward: a nation attempting to skate the long journey from curious novelty to genuine contender in record time. / Round Table's Happy Place (21:35)! On the show: Steve, Fei Fei & Yushan

China's aging society is turning to an innovative solution. The Time Bank allows volunteers to earn care credits by helping others today, securing support for their own future needs. This model transforms neighborly help into a sustainable system. How is this modern mutual aid network redefining elder care? On the show: Niu Honglin, Steve & Fei Fei

The global erosion of oral traditions continues not for lack of value, but because modern life leaves little room for slow, spoken memory. As languages vanish and elders pass on, entire ways of understanding the world disappear with them. Yet, the revival of Yimakan, from near-extinction to international recognition, demonstrates a powerful alternative. It shows that cultural heritage can survive not by being frozen in the past, but by finding a vital place in the present. / The joy and benefit of being bad at something (17:27). On the show: Niu Honglin, Steve & Fei Fei

What if artificial intelligence could step out of the screen and into the real world? This is not just about smarter conversation. This is Embodied Intelligence. Imagine a robot that does not just calculate a chess move, but physically makes it. Not every smart robot qualifies, so what changes when AI gains a physical form? On the show: Steve, Fei Fei & Yushun

When a global metropolis abandons de-icing agents, what happens? Beijing is finding out. To protect its environment, the city is swapping chemicals for manpower, trading spreaders for shovels. How can this modern capital handle winter the old-fashioned way? We examine the high-stakes experiment on Beijing's icy streets. / Is small talk dying (15:05)? On the show: Steve, Fei Fei & Yushun

For generations, the mercury thermometer has been a staple in Chinese households, its familiar ritual marking moments of care and concern. However, in accordance with the Minamata Convention and national environment and health policies, China is officially phasing out its production. On the show: Niu Honglin, Steve & Fei Fei

For decades, countries have invested heavily in engineering education. Despite this, many still face a persistent problem where graduates are theoretically proficient but lack practical readiness. This gap between academic training and industrial application is not unique to any one nation. China, however, is pursuing a distinct approach through its coordinated national strategy, the Outstanding Engineer training reform. This initiative aims to reshape how engineers are educated, with potential consequences for innovation worldwide./ Motivational Monday (18:41)! On the show: Niu Honglin, Steve & Fei Fei

With cold season here, our disposable mask defense is back. But when we toss it away, its story isn't over. It begins a centuries-long sentence in landfills and oceans. What if we could commute that sentence? Scientists may have found a way, transforming this symbol of waste into a tool for cleanup. Is this the key to breaking a toxic cycle? On the show: Steve, Fei Fei & Yushan

China's“strictest ever" e-bike safety standard is now the law of the land. Nearly 400 million rides are impacted. The mandate? Tougher frames, safer batteries—a complete upgrade for a new generation on two wheels. But what about the trusty bike in your garage? Will shops still service it? Will parts still exist? The transition has arrived. We break down exactly what it means for your daily ride and your wallet. On the show: Steve, Fei Fei & Yushan

China's logistics revolution is turning science fiction into reality, using unmanned technology like drones to deliver fresh produce to remote villages and reshape rural economies. Yet significant obstacles stand in the way. Can it clear these final barriers? We explore the promise and the challenges ahead. On the show: Steve, Fei Fei & Yushun

Plugging a colossal solar farm into the grid requires the precision of open-heart surgery on our power networks. The universal playbook engineers need for this task is now a reality. Forged by China and 13 other countries, this new global standard is the critical blueprint for a renewable future. / How do you stay warm and safe in deep winter (15:07)? On the show: Steve, Fei Fei & Yushun

China's monumental effort to plant nearly 200 billion trees since the 1970s is now facing a critical test. Scientists have discovered that these vast new forests, a cornerstone of the fight against climate change, are severely impacting water resources. This urgent challenge has spurred the country to lead a global shift toward "water-smart forestry," adapting its strategy for a world where every drop counts. On the show: Steve, Fei Fei & Yushan

How much of a nature survivalist are you? Extreme reality survival competitions have become more than just shows; they are now a cultural phenomenon, inspiring a generation to trade urban comforts for the challenges of the wilderness. These competitions are catalysts for a widespread movement, one that is actively redefining our ideas of adventure, self-reliance, and our fundamental relationship with the untamed power of nature. / Motivational Monday (21:09)! On the show: Steve, Fei Fei & Yushan

Fei-Fei Li is a Stanford professor, co-director of Stanford Institute for Human-Centered Artificial Intelligence, and co-founder of World Labs. She created ImageNet, the dataset that sparked the deep learning revolution. Justin Johnson is her former PhD student, ex-professor at Michigan, ex-Meta researcher, and now co-founder of World Labs.Together, they just launched Marble—the first model that generates explorable 3D worlds from text or images.In this episode Fei-Fei and Justin explore why spatial intelligence is fundamentally different from language, what's missing from current world models (hint: physics), and the architectural insight that transformers are actually set models, not sequence models. Resources:Follow Fei-Fei on X: https://x.com/drfeifeiFollow Justin on X: https://x.com/jcjohnssFollow Shawn on X: https://x.com/swyxFollow Alessio on X: https://x.com/fanahova Stay Updated:If you enjoyed this episode, please be sure to like, subscribe, and share with your friends.Follow a16z on X: https://x.com/a16zFollow a16z on LinkedIn:https://www.linkedin.com/company/a16zFollow the a16z Podcast on Spotify: https://open.spotify.com/show/5bC65RDvs3oxnLyqqvkUYXFollow the a16z Podcast on Apple Podcasts: https://podcasts.apple.com/us/podcast/a16z-podcast/id842818711Please note that the content here is for informational purposes only; should NOT be taken as legal, business, tax, or investment advice or be used to evaluate any investment or security; and is not directed at any investors or potential investors in any a16z fund. a16z and its affiliates may maintain investments in the companies discussed. For more details, please see http://a16z.com/disclosures. Stay Updated:Find a16z on XFind a16z on LinkedInListen to the a16z Podcast on SpotifyListen to the a16z Podcast on Apple PodcastsFollow our host: https://twitter.com/eriktorenberg Please note that the content here is for informational purposes only; should NOT be taken as legal, business, tax, or investment advice or be used to evaluate any investment or security; and is not directed at any investors or potential investors in any a16z fund. a16z and its affiliates may maintain investments in the companies discussed. For more details please see a16z.com/disclosures. Hosted by Simplecast, an AdsWizz company. See pcm.adswizz.com for information about our collection and use of personal data for advertising.

In China, a generation raised on digital savvy is rewriting the rules of consumption. Their new status symbol is one of financial intelligence: the pre-owned electric vehicle. Confronting a cooling used-car market and the steep depreciation of new energy vehicles, they see opportunity instead of crisis. They apply a hacker's mindset to mobility, finding luxury in efficiency and status in savvy calculation. On the show: Niu Honglin, Steve & Fei Fei

For many Chinese students, winter once meant chilly classrooms and bundled-up uniforms. Now, it brings free ski passes and extra family time. This new wave of "snow holidays" meets a generation's craving for movement, experience, and hands-on learning. As cities roll out offerings from free ski lessons to ice-fishing festivals, we are witnessing an ambitious attempt to transform snow into a genuine opportunity. / Have you stolen some sugarcane (17:58)? On the show: Niu Honglin, Steve & Fei Fei

Sensitive skin is now a widespread reality across China, with a booming care market standing at the intersection of hope and skepticism. As winter settles in, the hidden triggers behind the reactivity, the bold claims on every bottle, and the definitive science behind the persuasive spin all come into focus. On the show: Steve, Fei Fei & Yushun

How many times in your life do you retire? The traditional answer—once—is being overturned by a new wave of workers embracing months-long disconnections, or micro-retirements. This trend is a direct response to systemic burnout, prompting a vital debate on the real career risks of trading future security for present sanity. / The music to calm your pet (19:03). On the show: Steve, Fei Fei & Yushun

Gig work in China is entering a new phase. Leading delivery platforms are building affordable apartments for riders and introducing new welfare benefits. These measures offer the promise of security for millions. But the future of these improvements remains uncertain. The true test will be whether these benefits are a stable foundation or if their cost is quietly moved back onto workers through the platform's systems. On the show: Steve, Fei Fei & Yushan

Artificial intelligence now aids in diagnosing conditions such as depression within China's mental health system. By analyzing vocal patterns and micro-expressions, these technologies offer the potential for enhanced clinical efficiency and early intervention. Yet their arrival invites a deeper reflection on the nature of care itself, challenging us to balance the precision of algorithms with the irreplaceable role of human empathy. / Motivational Monday (20:28)! On the show: Steve, Fei Fei & Yushan

Dr. Fei-Fei Li is known as the “godmother of AI.” She's been at the center of AI's biggest breakthroughs for over two decades. She spearheaded ImageNet, the dataset that sparked the deep-learning revolution we're living right now, served as Google Cloud's Chief AI Scientist, directed Stanford's Artificial Intelligence Lab, and co-founded Stanford's Institute for Human-Centered AI. In this conversation, Fei-Fei shares the rarely told history of how we got here—including the wild fact that just nine years ago, calling yourself an AI company was basically a death sentence.We discuss:1. How ImageNet helped spark the AI explosion we're living through2. Why world models and spatial intelligence represent the next frontier in AI, beyond large language models3. Why Fei-Fei believes AI won't replace humans but will require us to take responsibility for ourselves4. The surprising applications of Marble, from movie production to psychological research5. Why robotics faces unique challenges compared with language models and what's needed to overcome them6. How to participate in AI regardless of your role—Brought to you by:Figma Make—A prompt-to-code tool for making ideas realJustworks—The all-in-one HR solution for managing your small business with confidenceSinch—Build messaging, email, and calling into your product—Transcript: https://www.lennysnewsletter.com/p/the-godmother-of-ai—My biggest takeaways (for paid newsletter subscribers):https://www.lennysnewsletter.com/i/178223233/my-biggest-takeaways-from-this-conversation—Where to find Dr. Fei-Fei Li• X: https://x.com/drfeifei• LinkedIn: https://www.linkedin.com/in/fei-fei-li-4541247• World Labs: https://www.worldlabs.ai—Where to find Lenny:• Newsletter: https://www.lennysnewsletter.com• X: https://twitter.com/lennysan• LinkedIn: https://www.linkedin.com/in/lennyrachitsky/—In this episode, we cover:(00:00) Introduction to Dr. Fei-Fei Li(05:31) The evolution of AI(09:37) The birth of ImageNet(17:25) The rise of deep learning(23:53) The future of AI and AGI(29:51) Introduction to world models(40:45) The bitter lesson in AI and robotics(48:02) Introducing Marble, a revolutionary product(51:00) Applications and use cases of Marble(01:01:01) The founder's journey and insights(01:10:05) Human-centered AI at Stanford(01:14:24) The role of AI in various professions(01:18:16) Conclusion and final thoughts—References: https://www.lennysnewsletter.com/p/the-godmother-of-ai—Production and marketing by https://penname.co/. For inquiries about sponsoring the podcast, email podcast@lennyrachitsky.com.—Lenny may be an investor in the companies discussed. To hear more, visit www.lennysnewsletter.com

Now on Spotify Video! As a Stanford AI scientist, Dr. Fei-Fei Li realized that artificial intelligence had advanced to a point where it was transforming society faster than most people could understand. Confronted with the ethical, social, and economic risks of this rapid growth, she felt a deep responsibility to guide AI toward serving humanity. This inspired her to co-found the Stanford Institute for Human-Centered AI, developing a framework that prioritizes humankind. In this episode, Dr. Fei-Fei shares how we can harness AI responsibly and design technology that enhances, not replaces, human potential. In this episode, Hala and Dr. Fei-Fei will discuss: (00:00) Introduction (02:33) The Evolution and Limits of Artificial Intelligence (09:56) How AI Models Like ChatGPT Are Trained (14:12) Dr. Fei-Fei's Journey and Responsibility in AI (19:15) How Computer Vision Brings AI to Life (25:59) Ethical AI, Human Dignity, and the Future of Work (32:57) The Three Pillars of Human-Centered AI (35:10) Confronting Fears of AI in Action (39:59) AI in Business: How Entrepreneurs Can Thrive  Dr. Fei-Fei Li is a professor of computer science at Stanford University and co-director of the Stanford Institute for Human-Centered AI. Her groundbreaking work in computer vision AI has shaped how machines see and understand the world. Dr. Fei-Fei is the author of The World's I See, a memoir that weaves together her personal journey with the history and development of artificial intelligence. Sponsored By: Indeed - Get a $75 sponsored job credit to boost your job's visibility at Indeed.com/PROFITING  Shopify - Start your $1/month trial at Shopify.com/profiting.  Quo - Get 20% off your first 6 months at Quo.com/PROFITING  Revolve - Head to REVOLVE.com/PROFITING and take 15% off your first order with code PROFITING  Merit Beauty - Go to meritbeauty.com to get your free signature makeup bag with your first order.  DeleteMe - Remove your personal data online. Get 20% off DeleteMe consumer plans at to joindeleteme.com/profiting  Spectrum Business - Visit Spectrum.com/FreeForLife to learn how you can get Business Internet Free Forever. Airbnb - Find yourself a cohost at airbnb.com/host  Resources Mentioned: Dr. Fei-Fei's Book, The Worlds I See: bit.ly/WorldsISee  Stanford Human-Centered AI Institute Website: hai.stanford.edu/  Active Deals - youngandprofiting.com/deals  Key YAP Links Reviews - ratethispodcast.com/yap YouTube - youtube.com/c/YoungandProfiting Newsletter - youngandprofiting.co/newsletter  LinkedIn - linkedin.com/in/htaha/ Instagram - instagram.com/yapwithhala/ Social + Podcast Services: yapmedia.com Transcripts - youngandprofiting.com/episodes-new  Entrepreneurship, Entrepreneurship Podcast, Business, Business Podcast, Self Improvement, Self-Improvement, Personal Development, Starting a Business, Strategy, Investing, Sales, Selling, Psychology, Productivity, Entrepreneurs, AI, Artificial Intelligence, Technology, Marketing, Negotiation, Money, Finance, Side Hustle, Startup, Mental Health, Career, Leadership, Mindset, Health, Growth Mindset, AI Marketing, Prompt, Generative AI, AI for Entrepreneurs, AI Podcast