Most substances contract when they cool down. The molecules slow, pack closer together, and take up less space. Water doesn’t do that. Once water gets below about 4°C, it starts expanding again. By the time it freezes solid, it’s about 9% bigger than the liquid water it came from.
The reason is the unusual shape of water molecules. Each water molecule has a kind of V shape, and when they freeze, they lock into a hexagonal crystal pattern that has a lot of empty space between molecules. That open structure makes ice less dense than liquid water - which is why ice floats.
This isn’t just a weird chemistry trick. It’s vital for life on Earth. Because ice floats, lakes and ponds freeze from the top down. The ice on top acts as an insulating blanket, keeping the water below liquid and warmer. Fish and other aquatic life can survive the winter underneath. If water acted like normal substances and froze from the bottom up, ponds and lakes would freeze solid every winter, killing most of the life in them.
