Once we have variables, we can use them to do some pretty interesting things. The most interesting thing is to build functions. Python functions are blocks of code that we can build to do a specific job. We build these functions once, and then we can reuse them in our code just by typing the name. This is really helpful. For example, if I need to write a program that adds two numbers (a calculator, for example), I do not want to have to write three or four lines of code every time I want to add two numbers. Instead, I want to write one function that can add two numbers together, and then use that single line whenever I need to add numbers.

Before we begin building functions of our own, we need to also know that Python has a lot of amazing functions that are built in. Some of Python's functions are things we will use all the time. Others we won't talk about in this book, but as you become a more skilled programmer, you will learn more about Python's built-in functions.