Another one:

Try to double-press (like you double-click) the start-stop button of your stopwatch as quickly as you can. This will obviously show the time elapsed between your first and second presses. I never got under 0.08s and usually around 0.12s. /msg me if you've got more and you want to make your ego feel better.

This is a good way to demonstrate how slow the human body really is, and also a good way to get round button marks on your fingers for the rest of the day, especially if your watch is one of those with buttons that actually hurt when you press them. For other methods, please visit Masochism.