By the way, I think writing or defacing money is illegal or probably illegal so I am NOT advocating it. Only pointing out that I have seen it done.
It's only illegal if done for fraud or if it renders the bill unfit for use. See http://www.lawforkids.org/speakup/view_question.cfm?id=407
If you feel like defacing some money, here are some interesting ways to do it: