<human>:  When a liquid crystal is cooled, it becomes a solid. Why?
<bot>: When a liquid crystal is cooled, it becomes a solid because the molecules in the liquid crystal begin to arrange themselves in a regular pattern. This regular pattern is called a crystal lattice. The molecules in the liquid crystal are arranged in a regular pattern because they are held together by weak forces called Van der Waals forces. These forces are very weak, so they are not strong enough to hold the molecules together when the liquid crystal is cooled.