from The Mozmonkey Blog

This year I decided to go all out for April Fools and do something that could be duplicated across multiple computers without permanent damage — and so the singing keyboard hack was born.

Concept

Take the musical element from a musical greeting card and connect it to the caps lock LED on the user’s keyboard. Each time the victim presses caps lock the music plays (and quite loud too). This is a great sleeping prank…It might be found on the first day, or weeks from implementation. It’s the perfect prank.

Why Caps?

True, most people don’t use caps lock very often, if ever, but unfortunately not many other keys are linked to an LED. Also most keys are only pressed momentarily, which would require a solid-state bounceless switch; which I had trouble getting to work. The num lock LED turns on and off several times during boot time, so that’s out and only leaves the caps lock as a viable solution.