Molecules do the same thing. They slow down and move closer together when they are cold. Ask the students to come into the circle and to huddle close together. When molecules get cold enough they freeze, although that does not mean they are totally without motion, even as the children will not be totally without motion. Explain that this is a solid.
Molecules are sometimes in between.
We call this state liquid. They spread out a little bit, but they stay inside their container. Have the students bounce slightly and push apart but stay inside the circle. Now you are ready for some exercise. Using the cues Hot, Warm, Cold, etc, or Solid, Liquid, Gas, have them show by their actions what the molecules would do in that state.