Solids have a definite form. That form usually doesn't change. Another form of matter is liquid. Liquid changes shape depending on what is holding it, but doesn't change size. The last form of matter is gas.
Gases have neither a definite shape nor size.
Then transfer that exact amount of air into a container. Explain what will happen to the water. Make a large chart divided into solids, liquids and gases. Have the students cut magazine pictures as homework and paste on during science. Copy master from Health Science, Grade 1, pp.