Give each child a sandwich baggie with an ice cube in it. Kids race to melt the ice cube in the bag, transforming their solid into a liquid. Once they have a liquid, the teacher adds a piece of an alka seltzer tablet to their bag - presto: gas!
So remembering this for whenever I have to caulk anything again: using an ice cube: The caulk will not stick to the ice, and the ice will melt to the shape of the joint to become the perfect shaped tool. Thanks Apartment Therapy!