點解湖底不結冰

攝氏四度的水密度最高，會沉下去。

When the temperature of  water decreases, there is fewer kinetic energy and H₂O molecules in liquid water takes up less space and density increases.

Below 4 degree of Celsius, however, the density decreases again.

Water and ice have the same network of bonds (hydrogen bond). Water at 25 degree of Celsius is so rapidly breaking bonds between H₂O units and reforming them that extra water molecules get trapped inside the water lattice. That's why water is more dense than ice?

The bonds in water break more slowly as temperature decreases and the structure tend to trap fewer extra water molecules. At low temperature, more of the water has the same lattice (crystal) as ice.

(Text is modified from butane.chem.uiuc.edu / Image from wtamu.edu)