Why does ice float?

Most things get smaller and heavier for their size when they freeze. Water is strange: when it freezes, it gets bigger.

Water is made of tiny molecules. In liquid water they move around freely and sit close together. When water freezes, the molecules lock into a pattern with spaces in it, like a honeycomb. The spaces make ice lighter than the same amount of liquid water.

Because ice is lighter for its size, it floats on top of water instead of sinking.

This strange habit keeps fish alive. In cold countries, lakes freeze from the top. The ice on top acts like a blanket, and the water underneath stays liquid all winter.