Science & TechFact
Water is heaviest at 4 degrees. That saves fish.
Almost every substance gets denser as it cools. Water only does so down to 4 °C. Below that, hydrogen bonds arrange the molecules into a looser lattice; ice is about 9 % less dense than liquid water and floats. Lakes therefore freeze from the top down while 4 °C water stays at the bottom.
Useful: That's why freezing water bursts pipes and bottles: it needs more room than before.
Source: en.wikipedia.org