Molecules are constantly moving because they have energy. In a liquid form, water molecules have more energy than in a solid – they move around quickly, essentially bouncing off of one another. As the liquid cools down, the amount of potential energy is reduced and the molecules start to move slower. When the water temperature reaches around 0°C, the molecules stick together and form a solid – ice. Even in this solid stage, the molecules are still moving – we just can’t see it.