Robert Paulus, age 8, of Tucson, Arizona, for his question:

Why do ice cubes float?

A cork floats because it is lighter than water. This means that a bucket full of water weighs more than a bucket full of cork. For its size, cork weighs less than the same amount of water. An ice cube floats because, for its size, it also weighs less than water. This is because something strange happens when liquid water freezes and becomes solid ice. It swells up, expands and takes up more room. This happens behind the scenes, down in the world of mini molecules, which are too small for our eyes to see.

All water molecules are alike. When they spread far apart, they form gaseous water vapor, which is light enough to float in the air. Liquid water is heavier than vapor because its molecules cling close together. When water freezes, its molecules stop moving around and arrange themselves in a solid formation. They do not fit perfectly together, so the solid ice is riddled with tiny spaces. These spaces make the ice bigger    but not heavier. So, for its size, an ice cube weighs less than water. It floats because it is lighter than the water.