Model exponential growth using pennies. Think of a penny as a single-celled organism that reproduces by dividing into two once each day. On day 1, you have one penny. On day 2, you have two pennies. Continue the exercise out to at least 10 days. How many pennies do you have at the end of that time? Now make a graph showing the number of pennies on the y-axis and the day on the x-axis. Does your graph look like the graph of exponential growth?