This part of the Kinetic Molecular Theory states that when heated, molecules get excited move more bouncing off of the walls of it containers, and eventually becomes a gas.  For example, in the picture to the left, imagine the bubble as molecules, and the molecules are moving so much, they're coming out of the liquid state, and becoming a gas.

 -Solids (left) retain it's own shape, and the molecules stick firmly together.  Since molecules are in constant motion, they are always vibrating, or bending.
 -Liquids (middle) retains the shape of the container it's held in, but the molecules are moving randomly and gliding over each other.
-Gases (right) Does not retain a shape but it's molecules move in straight lines.  The molecules bounce of each other, and the walls of its container, but no energy is lost.