In this activity, learners grow germs collected from their hands and other objects. They cultivate the germs on a growth medium (such as slices of grapefruit or processed cheese) for a week. Learners document the growth and compare how different sources produce different amounts of growth. The lesson plan addresses how doctors grow cultures of different germs to study them and to diagnose sick people. It also discusses the history of the discovery of germs, including key historical figures in the development of germ theory. Total learning time for this activity is under an hour, but it's spread across several sessions over at least a week. Learners gather samples in one session, and then examine them at least a week later.