We're gonna talk about e! The big famous constant e! Ok, it's one of the famous mathematical constants, one of the most important, goes along with pi, and I dunno, golden ratio, and square root of two. Constants in math that are the most important constants, and e is one of those constants. So e is an irrational number, and it's equal to…2.718281828 something, something, something. The problem with e, is it's not defined by geometry. Now pi, is a something that's defined by geometry, right? It's the ratio of a circle's circumference and it's diameter, and it's something the ancient Greeks knew about. And a lot of mathematical constants go back to the ancient Greeks, but e is different. e is not based on a shape, it's not based on geometry. It's a mathematical constant that is related to growth, and rate of change, but…why is it related to growth and rate of change? So let's look at the original problem where e was first used.