Why ice floats

Almost every substance becomes denser as it cools. Water does too, until it reaches about four degrees above freezing. Then it does something strange: it begins to expand again, and the solid it finally forms is lighter than the liquid it came from.

The reason lies in the shape of the molecule. A water molecule is bent, with its two hydrogen atoms at an angle, and the atoms attract their neighbours. As water cools, the molecules slow down enough for these attractions to lock them into a rigid six-sided pattern. That pattern has gaps in it. The molecules in ice are held slightly further apart than they were when sliding freely in the liquid, so the same number of molecules takes up more room.

The consequence is enormous and easy to overlook. Because ice floats, a lake freezes from the top down. The sheet of ice that forms acts as a lid, insulating the water underneath, and fish survive the winter in liquid water below. If ice sank, each piece would drop to the bottom out of reach of the sun, and lakes in cold regions would freeze solid from the bottom up and stay frozen.

It is worth pausing on how narrow this is. The behaviour follows from the angle between two hydrogen atoms. Change that angle and the ponds of the northern world become blocks of ice with nothing living in them.