What is needed
The monitor redraws the screen from top to bottom at a rate near 60 Hz. This means that it will act like a strobe light, and allow you to "stop" fast motion. But since it is drawn from top to bottom, it will stop the motion at different points! So it will look like the pencil has curved: the top will be stopped at one time, and the bottom at another time!
When you hold the pencil horizontally, this does not happen. Why not?
(Note: we have been doing this with Mac monitors, where it works really well. We are not sure it works with PCs too, but we guess that it does. If you find any cool or unusual effects, let us know! You can write to us using the form here.)