Hey, smart people.

Joe here.

I'm willing to bet you're familiar with this stuff-- ice, good ol' frozen H2O.

But have you ever really stopped to think about it?

I mean, sure, we put it in our drinks, we slip on it, we slide on it, we skate on it, and we're worried climate change is going to melt every last bit of it.

But even though ice is such a common and ordinary substance, it still has a few weird properties and mysteries that we're just now figuring out.

And one of those that we're just now learning the secrets to is one of the most obvious questions about ice-- why is ice slippery?

[MUSIC PLAYING] The most intuitive explanation for ice's slipperiness is that there must be a thin layer of liquid water on the surface of the ice.

This is an idea that goes back to the 1840s when the physicist Michael Faraday did this.

He took two ice cubes.

When they were placed in contact for just a few seconds, they became one ice cube.

Now Faraday thought that there must be a layer of liquid water between them that froze.

But I don't see a layer of liquid water.

But maybe something has to create the layer of liquid water.

This is what's called a phase diagram, and this one is for water.

For a particular temperature and pressure, it tells you which phase-- solid, liquid, or gas-- the substance will be in.

Looking at the diagram, we see water at minus 5 degrees Celsius and 1 atmosphere of pressure, like your typical ice rink, will be solid ice.

This phase diagram also shows us something else.

There's more than one way to melt ice.

You can raise the temperature.

We're all familiar with that way of melting.

But water is special because you can also melt it by increasing the pressure.

For most substances, their solid form is more dense than the liquid form.

Think of candle wax.

So putting the solid form under more pressure only makes it more solid.

But water ice is different and weird.

Its liquid form is more dense than the solid form.

And that's why ice floats.

So if we take solid ice and squeeze it under a lot of pressure, when we force it to be more dense, it can become a liquid again, even at much colder temperatures where we'd usually expect it to be a solid.

So the idea is this.

When you step onto the ice, you exert a pressure on the area under your feet that lowers the melting point of the ice and creates a thin layer of liquid slippery water.

Now pressure does lower the melting point of ice, and for a long time, people thought that this pressure melting was the reason ice was slippery.

A person's weight concentrated on the area under their feet melted the top layer of the ice.

But something doesn't add up here.

I weigh about 88 kilograms.

And the area under these skates is like 900 square millimeters.

You plug that into the appropriate equations, and it turns out it only lowers the melting point of ice by a fraction of 1 degree Celsius.

And if you're wearing regular shoes, you've lowered the melting point by even less.

Basically for pressure melting to work, you'd have to be an elephant in stiletto heels.

Look at that phase diagram again.

To get the kind of pressure on ice skates needed to lower the melting point of water by any significant amount, I need to weigh over 3,000 kilograms.

That's only half as heavy as most elephants.

There's another answer that feels closer to the truth: friction.

As my ice skates are gliding across the ice, they create friction.

That friction creates heat, and that can create a thin layer of liquid water on top of the ice.

Well, we know this does happen.

Ice skates gliding across ice can create friction that can melt the ice beneath them.

But it also can't be the whole story, because anyone who's tried skating for the first time knows you don't have to move very much for your feet to slip out from under you.

And it also doesn't explain why you don't have to move for ice to be slippery.

So does friction explain it?

Well, maybe in part, but it doesn't explain the whole story of why ice is slippery.

There's a fundamental issue here that we haven't talked about.

Imagine a layer of water on some hard surface, like a puddle on a smooth floor.

It's pretty obvious that ice is actually way more slippery than just water.

There must be something else going on.

Several theoretical and experimental studies in the last few years have found the real cause of ice's slipperiness.

That thin layer of liquid water on the surface, it isn't really liquid water at all, but it's also not solid ice.

It's something very different and very strange.

OK, do you remember the weirdest property of solid water?

It floats.

Water's solid form is less dense than its liquid form.

I mean, think about it.

That is weird.

Solid rocks don't float on lava.

But now let's think about this on the molecular level.

Water is a polar molecule.

The slightly positive hydrogens are attracted to the slightly negative oxygens.

It's known as hydrogen bonding.

A single molecule in liquid water will only be hydrogen bonded to a few other water molecules, but a single water molecule in ice will have more molecules hydrogen bonded to it than in liquid water.

This makes ice a crystalline solid with water molecules bonded together, basically trapped in a very consistent, repeated pattern.

But what about a molecule on the surface of the ice?

Those may be bonded to as few as just one other molecule in the crystal.

And it's this lack of structure that allows the surface molecules to basically tumble around in disorder, detaching and then reattaching themselves to the surface and to each other.

Think about it like this.

These water molecules down here represent those deeper in the ice.

They're held in place pretty rigidly by all the hydrogen bonds surrounding them.

But up here on the surface, these water molecules don't have as many hydrogen bonds holding them in place.

And they're able to sort of roll around.

This creates something on the surface that isn't really a liquid, because water's phase diagram says that, technically speaking, water is solid under these conditions.

But it's not totally a solid either, at least not the way that we normally think of solids.

Scientists call it a quasi-liquid layer.

One researcher who studied its properties compared it to marbles scattered across a dance floor.

You just take one step, and the marbles roll out from under you.

Scientists still have a lot of questions about the properties of this quasi-liquid layer since the border between it and ice is really hard to see.

We're not even sure exactly how thick it is, somewhere between the size of a single bacterium and 1,000 times smaller than that.

We're talking a few molecules.

So ice is slippery because of a non-solid quasi-liquid layer of water on the surface.

It's not caused by pressure.

It's not totally caused by friction.

It's caused by the H2O molecules coming loose on the molecular level.

And those H2O molecules, they don't act like liquid water.

They act like something much weirder.

I know it's strange to say here in the 21st century that we don't know everything that there is to know about something as ordinary as frozen water.

But I think this demonstrates something really fundamental about science, that the more resolution they're able to gaze into the universe, whether it's on the biggest or the smallest, little molecular scales, well, we'll always keep uncovering new discoveries.

And we'll always keep updating the things that we knew yesterday, even if it's in the places that we've been looking all along.

Stay curious.