When we work with circles, one of the most popular things to pop up is pi. We know that for any circle the circumference divided by the radius gives pi, without failure always. Pi is a number that’s been found. e is the number that commonly pops up for rates of change. More specifically, it is used for when a population is growing based off the size of itself. So for example, as animals reproduce, more and more become able to reproduce, meaning more and more etc. This rate is an exponential rate, with e as the base.