Have them observe the balloon. Let the students move the bottle back and forth as time allows, and encourage predicting what will happen when they do. Ask for possible explanations and accept them all as something to consider. Explain that warm molecules are very active and spread out away from other molecules. Explain that molecules which bounce all over are a gas.
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.