Know that calculus is the study of how things are changing. Calculus is a branch of mathematics that looks at numbers and lines, usually from the real world, and maps out how they are changing. While this might not seem useful at first, calculus is one of the most widely used branches of mathematics in the world. Imagine having the tools to examine how quickly your business is growing at any time, or plotting the course of a spaceship and how fast it is burning fuel. Calculus is an important tool in engineering, economics, statistics, chemistry, and physics, and has helped create many real-world inventions and discoveries.