Episode Summary
Executive Summary: Hannah Fry and Michael Stevens explore the surprising science of water, from its molecular composition and role in life to its distribution on Earth. They discuss whether water itself is wet, the mineral content of tap water, the origins of Earth's water, and the dangers of overhydration. The episode also touches on water's geological classification and its importance in everything from making bagels to beer.
Main Topics: Is Water Wet? (Priority: 1/5): Debate on whether water itself is wet. Michael argues water molecules wet each other through surface tension, while Hannah suggests wetness requires contact with another material. Water Composition and Mineral Content (Priority: 4/5): Tap water is a 'rock smoothie' with dissolved minerals that vary by location, affecting taste, baking, and even human identification. Different regions produce distinct water with unique isotope signatures. Earth's Water Scarcity and Distribution (Priority: 5/5): Only 2.5% of Earth's water is fresh, and most is frozen or underground. Surface fresh water is a tiny fraction—0.0072% of all water. Water likely arrived via comets and is older than the Sun. Water as Lava and Ice as Rock (Priority: 3/5): Ice is a mineral, so melted ice is molten rock (lava). Water is therefore lava at room temperature. Fluid dynamics of lava and water are the same. Water Poisoning and Homeostasis (Priority: 4/5): Drinking too much water can strip electrolytes, causing confusion, hallucinations, and death. LD50 in rats is 90g of water per kg of body mass; humans can die after 6-7 liters in 3 hours. Industrial Water Reuse (Priority: 2/5): Singapore recycles sewage into ultra-pure water for semiconductor manufacturing, not drinking. The ISS also recycles water. Pure water can be dangerous if it strips the body of minerals.
Key Arguments: Water's wetness is a philosophical and scientific question: Michael argues water wets itself via surface tension; Hannah sees wetness as requiring a separate material. Earth's water is ancient and primarily from comets, with most being salty or inaccessible. The tiny fraction we use is vital. The etymology of 'loser' connects to 'dissolve'—problems disappear when 'dissolved' in a solution, and life depends on dissolved minerals in water.
Data Points: Water content in Diet Coke: 99% - Diet Coke is nearly pure water due to aspartame's potency, contrasting with regular Coca-Cola at 90% water. Fresh water on Earth: 2.5% - Only 2.5% of Earth's water is fresh, with most frozen or underground. Surface fresh water is 0.0072% of all water. Water's LD50 in rats: 90 g/kg body mass - For rats, 90 grams of water per kilogram of body mass is lethal to 50% of the population. For humans, 6-7 liters in 3 hours can be fatal. Water volume on Earth scaled to a 30 cm globe: 14 mL - Less than a tablespoon of water on a classroom globe represents all of Earth's water (including oceans, ice, etc.). Water older than the Sun: More than half of Earth's water - Water predates the Solar System; evidence from deuterium ratios and the Sun's formation supports this.
Pivotal Quotes: "One molecule of water dry. Two or more? Soaking wet." — Michael Stevens: Michael's definition of wetness: water becomes wet through intermolecular forces between molecules. "Tap water is just basically like a rock smoothie." — Michael Stevens: Tap water contains dissolved minerals that affect taste and properties. "We didn't leave the water behind us. All we did was we just turned ourselves into water balloons and carried it with us as we go." — Hannah Fry: Humans and land animals are essentially water balloons, carrying water from our aquatic origins.
Implications: This episode challenges listeners to rethink water as a scarce, ancient, and chemically active substance. The water cycle, distribution, and purity have direct implications for health, industry, and climate policy. Understanding water's role in biology and geology shifts the perception of Earth as a 'wet planet' to one with critically limited accessible fresh water.
From the Transcript
Hello and welcome to The Rest is Science. I am Michael Stevens. And I'm Hannah Fry. And today we're going to get a little bit splashy, a little bit wet. No, but we are going to talk about something that's wet. Water. Hang on. Are you sure water's wet? Actually, I'm not so sure. This is quite a big question. It's a very memed question. Is water wet? I have an opinion on it. Do you? I make one. You tell me yours. Okay, what I think is that one molecule of water dry. Okay. Answers more. Soaking wet. Yeah. Well, hold on. We should say, we should say first about why this is a memed question, right? Because it's like water itself, the things that it touches are wet, but is the water itself wet? Yes, that's right. Water makes things wet, but water on its own, would you call it wet? I would, because I think water can wet itself. Wetness is about intermolecular forces on water attracting themselves onto some other material. Okay. That other material, in my opinion,
To say that tap water is just basically like a rock smoothie, you know? Like, don't think of water as though it's just water. This has got all kinds of stuff in there. It's, yeah. I mean, there's calcium, there's magnesium, there's sodium, there's potassium, there's oleums. LA's water is very hard. Is it? It's got a lot of minerals in it compared to, say, the water in the Catskills, which provides water to New York City. In fact, there's a theory that it's that water that New York City gets from the Catskills that creates for the Them, their unique, special, one-of-a-kind pizza doughs and bagels. Makes it taste different. It makes it taste different. And this isn't just some kind of guess. It really is true that those minerals affect how the gluten forms, it affects how the flour sticks together. And so there are companies that will either truck New York City tap water to your business, whether it be in Florida or Philadelphia. There are also companies that replicate New York City's tap water off-site by adding in minerals.
You know, every breath you take contains some atoms that Julius Caesar breathed in. Here's the thing I like to think, right? In our evolutionary past, I mean, we started off in the water, and then it was one point where, you know, started laying eggs on land and kind of eventually evolved into creatures that could live predominantly on land. You know, we didn't leave the water behind us. All we did was we just turned ourselves into water balloons and carried it with us as we go. That's right, isn't it? And like, as you're, you know, drinking your drink during the day, what you're doing is you're just topping up the evaporation that you're. We like to call ourselves land animals, but really we're water balloons. In fact, the first living things to move onto land would do it, but then they'd go back into the water to lay their eggs. But there was a major evolutionary step where animals evolved that laid their eggs on land, and they needed to have a hard shell for that so that the water wouldn't all evaporate away. Exactly. They needed to evolve water balloon production reproduction organs, and they did.
About The Rest is Science
Join mathematician Professor Hannah Fry and science creator Michael Stevens (Vsauce) as they dig into the weird scientific questions that often go unexplored. Welcome to The Rest Is Science, a show that sits in the fascinating space between what we think we know, and what we actually know. Why do we assume we understand things like time, randomness, or even gravity? Once you start questioning these familiar ideas, reality becomes astonishingly strange and completely fragile. Whether you're a lifelong science fan or just naturally curious, The Rest Is Science will change your perception of reality, and prove that the biggest questions are always the most fun.