Gale Gilbert, age 12, of Houston, Tex., for her question:

Why does ice expand?

The molecules of a gas are widely separated. In a liquid, they are close enough to touch and as the liquid cools to a solid, its molecules become locked together. Most substances shrink as they freeze solid. Water is an exception. The reason for this lies in the odd shape of water molecules.

When water freezes, its molecules also lock together, but because of their odd shapes, they cannot fit together in a dense solid. They form a delicate lattice, riddled with miniature spaces. Countless numbers of these spaces add bulk to the solid and the ice needs more room. Ice expands because it is riddled with miniature holes not present in liquid water.