The science behind why water is wet

HostIt's one of those things we never really think about because it's just so basic. You jump in a pool or get caught in the rain, and you're wet. It's just how the world works. But when you actually try to pin down what wetness is, it gets a bit weird. Is the water itself wet, or does it just make other things wet? What's actually happening when we feel that splash?

GuestIt's a bit of a brain teaser, honestly. Most of us think of wet as a thing water just is. But in the world of science, wetness is more about how a liquid acts when it touches a solid surface. If you want to get really into the weeds, you could argue that water itself isn't actually wet. It's only the thing that makes other stuff wet. To be wet, a surface has to be covered in a thin layer of liquid that clings to it. So, a single tiny bit of water, the smallest piece you can have, wouldn't be wet on its own. It needs to be part of a group that can coat something else.

HostThat sounds like a bit of a trick. If I have a glass of water, and I put my finger in it, my finger gets wet. Are you saying the water in the glass isn't wet until I touch it?

GuestWell, it depends on who you ask, but look at it this way. Wetness comes from two different forces playing a game of tug-of-war. One force is how much the water bits want to stick to each other. They're very clingy. They have a plus side and a minus side, sort of like magnets, so they're always trying to huddle together. That's why you see raindrops bead up on a window. They would rather hang onto each other than the glass. But the other force is how much those water bits want to stick to other things, like your skin or a towel. When the pull toward the towel is stronger than the pull toward the other water bits, the water spreads out. It coats the fibers. That spreading and sticking is what we call wetness.

HostSo if the water bits liked each other too much, they would never spread out, and nothing would ever get wet?

GuestThat's pretty much it. Think about a bead of mercury. If you spill it, it stays in these tight little silver balls. It doesn't spread out or soak in. It's a liquid, but it's not wet in the way we usually mean. Water is special because it's right in the sweet spot. It sticks to itself well enough to stay a liquid, but it also really likes to grab onto other things. It wants to climb into the tiny gaps in your clothes or the pores in your skin.

HostBut wait, I have seen water bead up on a freshly waxed car. It just sits there in little drops and rolls right off. In that case, is the water failing to be wet, or is the car just refusing to get wet?

GuestIt's the car. Or rather, it's the wax. Some materials are what we call water-fearing. Their surface is so smooth or made of stuff that water bits just can't grab onto. Since the water can't find a handhold on the wax, it just pulls back into its own little huddle. It forms those round drops because it would rather touch itself than the car. So, in that moment, the waxed car isn't wet. Even though there's water sitting right on top of it, the water isn't coating the surface. It's just resting there.

HostOkay, that makes sense for a car. But what about us? When I feel wetness, it feels like a very specific thing. It's not just sticking. It's a whole feeling. How does my body actually know when it's wet?

GuestThis is the part that blows most people's minds. Humans don't actually have a sense for wetness. We have nerves for heat and cold, and we have nerves for touch and pressure, but we don't have a nerve that just screams wet.

HostHold on, I know when my socks are damp. I don't have to look at them to know. If I don't have a wetness sensor, how am I feeling that?

GuestYour brain is basically a detective. It takes a few different clues and puts them together to make a guess. The main clue is cold. Because water takes away heat from your skin much faster than air does, your cold sensors start firing. The second clue is the way the water moves or pushes against your skin. Your brain takes that feeling of cold plus that feeling of something sliding or pressing, and it says, okay, this must be wet. If you want to test this, you can put on a thin latex glove and dip your hand in cold water. Even though your hand is bone dry, it'll feel wet because your brain is getting the cold and the pressure at the same time. It can be fooled quite easily.

HostThat's a bit unsettling. It feels like my brain is just making up a reality because it doesn't have the right tools. But if I'm in a bath, the water is warm. I don't feel cold, but I still feel wet. Where's the clue there?

GuestIn that case, it's mostly about the pressure and the way the water moves against your tiny hairs. Also, your brain knows you're in a bath. It uses what it sees and what it expects to help fill in the gaps. But even then, if the water is exactly the same temperature as your skin and you stay perfectly still, you might lose that sharp feeling of being wet. It all starts to blur together. We're very good at sensing when things change, but not so good at sensing when things are steady.

HostSo, it's about the water sticking to things and our brain guessing based on the cold and the touch. But what about the water itself? Is a single water bit, a molecule, wet?

GuestMost scientists would say no. Wetness is a team sport. It's a property that only shows up when you have a big group of molecules working together. A single molecule can't flow, it can't coat a surface, and it can't bead up. It's like asking if one person can be a crowd. You need the crowd to have the behavior of a crowd.

HostWater needs its friends to be itself. It's funny to think that the most common thing in our lives is actually just a bunch of tiny magnets playing a game of tag with our skin.

GuestIt really is, and we're still learning the tiny details of how water interacts with different materials at that very small scale.

HostThose tiny magnets are the reason the rain sticks to the window instead of just bouncing off like a rubber ball.

GuestThat pull is what keeps the world from being a very dry, very dusty place.

HostThe next time I step in a puddle, I'll try to remember it's just my brain playing a trick with the cold and the splash.