You wake up and head to school feeling fine. By lunch you're starting to feel icky and by the end of school you're sick.
Under the right circumstances, bacteria can reproduce every 20 minutes.
Have your students determine how many bacteria will exist after 10 hours, if they begin with just one. They'll have to do their calculations in their head/on paper, as a calculator will quickly become useless!
Scientists believe that humans can start to feel the effect of harmful bacteria when there are about 2000 bacteria in the body. How many minutes/hours did it take to reach this number?
If time permits and you're willing, you could use popcorn kernals to model the number of bacteria (up to a certain point....). Have your students count out lots of 10 kernals in small cups - it makes it easier to count out 100, etc.