The usual rule
Cool most substances and their particles slow, sit closer together and take up less space. The solid is denser than the liquid, so it sinks.
Water breaks the rule
As water freezes, its molecules lock into an open six-sided lattice held apart by hydrogen bonds. That structure has gaps, so ice is about 9% less dense than liquid water.
Why it matters to living things
Ice forms a lid on a pond instead of filling it from the bottom up. Water below stays liquid, and fish, insects and plants survive the winter under it.