Physically I changed the dollar but it can still be used. Even if you tear a bill in half, as long as you tape it back together, it can still be used as long as the serial numbers match. You can also draw and write on a bill as long as the serial numbers aren't covered. The only things you are not allowed to do is change the denomination, like adding to zeros to a dollar bill and passing it off as a hundred. Also burn, shred, or destroy currency in which it renders it unfit for circulation. Lastly you can't advertise a business on a bill. I just thought it was interesting that you can make a lot of changes to a dollar but it's still worth the same in the end. I also wrote 'Where's George?' an the bill because www.wheresgoerge.com is a currency tracking project. So you can track US or Canadian dollar bills as they travel across the country.