Water is the only substance that expands when it freezes — and this accident makes life possible

Almost all substances contract as they cool and solidify. Water is a striking exception: ice is less dense than liquid water, which is why it floats. This anomaly arises from the geometry of hydrogen bonds forming hexagonal crystals. Its consequence is profound: ice floats on top of lakes and oceans rather than sinking, insulating the liquid water beneath and allowing aquatic life to survive winter. If water behaved like most substances, lakes would freeze solid from the bottom up, killing everything. The anomaly of water is one of the reasons Earth can sustain life at all.

Source: Ball, P., H2O: A Biography of Water (Weidenfeld & Nicolson, 1999)