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.

The reason we can gracefully glide on an ice-skating rink or clumsily slip on an icy sidewalk is that the surface of ice is coated by a thin watery layer. Scientists generally agree that this ...