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Olivia: Have you ever been walking on a snowy sidewalk on a freezing day, stepped on a hidden patch of ice, slipped, and fell flat on your butt? Scientists seem to agree that what’s causing the slipperiness is a really thin layer of liquid water on top of the ice, but they’re not entirely sure how it forms.

Most solids don’t have such a layer, but ice isn’t like most solids. So researchers have come up with a couple ideas, involving pressure, friction, and just how the water molecules interact. For decades, people have thought that you can exert pressure on ice to melt the top layer a tiny bit, like when you’re ice skating.

This could happen because of one of water’s weird properties: ice is less dense than liquid water. Thermodynamically, when you put extra pressure on ice – like right under the blade of a skate – the system tries to lower that pressure again by decreasing the volume. Since liquid water takes up less space than ice, its melting point drops a bit, so the solid can melt a little, and you slide across. And once your skate passes, the water refreezes.

So that seems to make sense, but it doesn’t completely check out. Even for a heavier skater, the melting point would only lower by a few degrees at most, which means really cold ice would stay frozen. Plus, a person wearing normal shoes, which put less pressure on the ice than a thin ice skate blade, will still slip.

So another possibility is that the friction from your shoe rubbing against the ice creates enough heat to melt it. And while that’s true, ice is still slippery when you’re standing still. So that explanation doesn’t make the cut either. But there’s a third idea, based on an observation the physicist Michael Faraday made in 1850: He pressed two ice cubes against each other and saw that they froze together. And he figured that the liquid surface layers became solid when they weren’t touching air anymore.

This led modern scientists to look into an idea called surface melting – maybe water molecules on the surface of ice can move around more than the ones inside, since there are no molecules above them to help hold them in place. Because these surface molecules are less stable, they have enough energy to make a liquid-like layer even at below-freezing temperatures. In simpler words: ice is just inherently slippery.

None of these explanations have been completely proven or disproven, so scientists think a combination of them and the weirdness of water are all at play. So the next time a patch of ice takes your feet out from under you, you can remember that ice is a pretty cool solid.

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