A thin, watery layer coating the surface of ice is what makes it slick. Despite a great deal of theorizing over the centuries, though, it isn't entirely clear why that layer forms.

When you step onto an icy sidewalk or push off on skis, the surface can seem to vanish beneath you. For more than a century, scientists have debated why ice stays slippery, even well below freezing.

It’s an oft-cited science “fact” that ice is slippery due to pressure or friction, but this explanation doesn’t explain why ice’s slippery behavior remains at temperatures where such melting isn’t ...