Read The Germ Busters by Rosemary Wells. Discuss the importance of washing hands to stay healthy. Take three slices of bread. As the teacher make sure that you have freshly washed hands. Hold the bread in your clean hands then put it in a zip lock bag marked "clean hands". Then have children who have slightly dirty hands or maybe don't look dirty touch another slice of bread and put it in the bag you marked "not too dirty. Then with a third slice of bread have the kids whose hands are visibly dirty touch it and mark the bag "really dirty". Ask the kids to guess what will happen to the three slices of bread. Have them record their guess. Keep the bread slices where they can be observed and check them weekly. What happend? How did it compare with their guesses?
I think this would be great to do right after recess.