This is an experiment that will teach children "why" we must recycle. It can used by teachers as a project or by parents.
Have half of the class fill styrofaom cups with plain dirt and small pieces of broken up styrofoam from another cup. Keep it watered as you would a plant.
Have the other half of the class fill foam cups with plain dirt and small pieces of plain notebook paper or newspaper. Keep the soil damp.
After a few weeks when the children examine their cups ,and stir the dirt, the paper will have nearly dissolved, but the foam pieces will still be there.