Ice floats on liquid water because freezing makes it slightly less dense than the liquid.

The longer version

When water molecules lose enough heat they lock into a fixed pattern held together by hydrogen bonds. That pattern keeps them slightly farther apart than they sit in the liquid, so the same mass spreads over more space. Density compares mass with volume, and spreading the same mass over a bigger volume lowers the value.

The result is a solid that weighs less per cup than the liquid around it, so it rises to the top. This is unusual, since most substances pack tighter as they solidify and sink in their own melt. Because ice stays on the surface, it also shields the water below from cold air.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.