Why does ice float on water?

Because ice is less dense than liquid water, which is unusual. Most substances become denser when they freeze.

When water freezes, its molecules arrange themselves into an open, hexagonal pattern held together by hydrogen bonds, which takes up more space than in liquid water. So ice is about 9 percent less dense and floats.

This matters for life on Earth. Lakes and ponds freeze from the top, and the ice layer insulates the water below, allowing fish and other creatures to survive the winter.