Letter on Something You Ought Not Write On!

So for this swap, you'll be writing your letter about anything you want, any length you desire. But you'll have to get creative and write it on something you really ought not to write a letter on.

This can be something big, small, but should be flat. Some ideas: the back of an exam, junk mail, receipts from the grocery, napkins, the margins of book pages, the back of an old photo, a paper bag, a piece of cardboard....

I'm sure you guys can think of more creative ideas than I can! ;)