I taught this to my first graders last year and they loved this activity. On our floor I taped out 3 sides of a square, large enough that about half my students could stand it in. Then we talked about the molecules in the different states of matter. My kids packed in the "jar" really tightly for solids, swayed side to side for liquids, and bounced off each other and out of the "jar" for gasses.
There's also a really cute book, I believe it's called "What's The Matter" and it gives lots of examples and explanations that children are familiar with.
Hope this helps!!
