What is an atom? When you hear this question, a picture like this probably comes to mind:

Here, we have "something" going around and around "something" else, almost like a little solar system.

To start off, let's be sure we understand what these "somethings" are. One is an electron, and one is the nucleus. Which is which?

Green is the nucleus, red is the electron
Green is the electron, red is the nucleus