Sixth graders will simulate the spread of a virus such as HIV through a population by "sharing" (but not drinking) the water in a plastic cup with several classmates. Although invisible, the water in a few of the cups will already be tainted with the "virus" (sodium carbonate). After all the students have shared their liquids, the contents of the cups will be tested for the virus with phenolphthalein, a chemical that causes a striking color change in the presence of sodium carbonate.