This isn’t something you’d normally find in a math class, but I find it incredibly useful to know how to turn a simple sheet of paper into the envelope you can mail it in. The letter you write goes on one side, and the address and stamp go on the other. Try it and see how you like it!
Because of of all the folding, the envelope is actually quite small when you’re done with it. If you want a regular-size envelope, tape two sheets of paper together to get a big 11×17″ sheet.