Timelapse footage of cubes of ice in a beaker melting. Below zero degrees Celsius, the average kinetic energy of the water molecules is low enough that they can form stable intermolecular hydrogen bonds, which lock them in a crystalline form. When the temperature rises, the molecules have enough energy to overcome these individual attractions, and the molecules move further apart, becoming a liquid. Even as a liquid the molecules attract each other, but the hydrogen bonds between any two molecules are fleeting, before they move apart and form similar bonds with other molecules.