Water expands when it freezes, which is why ice cubes float in a glass.

The longer version

Most substances contract as they solidify, but water does the opposite. As liquid water cools, molecules begin arranging into a hexagonal crystal held together by hydrogen bonds, and that open lattice occupies more space than the jumbled liquid did. The result is a solid roughly a tenth less dense than the water it came from.

This anomaly matters well beyond the kitchen. Floating ice insulates the water below it, and water seeping into cracks can widen them as it freezes, gradually breaking rock and pavement apart. Without this behavior, ponds would freeze from the bottom up and aquatic life would face very different conditions.

Where this entry sits

Everyday ingredients turn out to be full of quiet chemistry and long history. This collection gathers short, plain explanations of how bread rises, why onions sting, where common spices originate, and how coffee, tea, and chocolate reached the modern kitchen. The full list sits on the Food and Drink page, where every entry is listed in order.