In this class activity, students will each pick up a cup with a number on it. They will record this number on their worksheet. All of the cups are full of water except one cup number has a weak amount of sodium hydroxide. After you've demonstrated a "switch-a-roo" which is basically pouring the liquids back and forth with another student, students will then record who they made their "switch-a-roo" with. After 4 "switch-a-roo's" I walk around and put a few drops of phenothalein into their cups. If they're "infected" their cup will turn pink. If they're not infected, it will remain clear. This is an excellent way to get students to understand how scientists have to work backwards to find the source of the bacteria/virus and how quickly it can spread.