Using marbles to represent molecules, and shooting them at each other, you will see that when the force is increased, the marbles knock each other further apart.

Molecules in a material behave in a similar way when heat (the force) is added. For example, when a glass of water is heated, the molecules of water hit each other and some
molecules strike with such force that they bounce right out of the glass and into the air. This process is what scientists call "evaporation."

When the molecules in a material are very far apart, the material is called a gas.

When some heat is taken away from a material, the molecules do not hit each other with as much force. Thus, the molecules in the material would move more slowly and be closer together. When the molecules are in such a state, the material is called a liquid. This process is what scientists call "condensation."

When still more heat is taken away, the molecules in a material move even more slowly and strike each other with even less force. Thus, the molecules in the material are even more close together. When the molecules are in such a state, the material is called a solid.

Use this idea to explain the presence of solid wax at the base of a burning candle.

Other changes of state, such as melting, freezing, subliming, and frosting, can be explained using this model.