What is needed

* Pencil, Pen or any object similarly shaped

* Take the pencil in one of your hands and hold it tightly between your thumb and forefinger, in front of your monitor. Hold the pencil so it is vertical, very close to the monitor.
* Now wave the the pencil back and forth in front of the monitor. Does it stay straight? Does it curve? Does speed of movement change the pencil's behavior?
* Now hold the pencil horizontally and repeat the above step. Does anything happen?
* Try your own experiments!

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: This was done with Mac monitors, where it works really well. It should work with PCs too, so try it out.)