By Harold McGee
At a temperature that is characteristic of each solid substance, the rotation and vibration of individual molecules in that substance becomes forceful enough that the electrical forces holding them in place are overpowered. The fixed structure then breaks up, leaving the molecules free to move from one place to another. However, most of the molecules are still moving slowly enough that they remain influenced by the forces that once immobilized them, and so they remain loosely associated with each other. They’re free to move, but move together. This fluid but cohesive phase is a liquid.