"This is the grossest experiment I did while teaching about germs. Use three pieces of bread. You let all the kids see you put a piece of bread in a baggy with a glove on hence "controlled" then you wash your hands and put a piece of bread in a baggy for "clean" last, pass a piece of bread around and let every kid in class touch it then you put it in a baggy and label it dirty. Watch how the bread changes over time. A great way to teach the importance of hand washing."