At a Glance - Dimensions and Extensions
So far everything we've been doing has been flat, boring, two-dimensional. It's time to add some depth to things (and to show how hard it is to draw depth on a sheet of paper or a computer screen).
As you've hopefully noticed at some point in your life, the world is more than a single sheet of paper. In fact, you can have several different sheets of paper, oriented in several different ways. But this is about geometry, not the state of your backpack.