Here's an idea for a prank on your co-worker or friend; hack their keyboard to sing. 

The concept is to "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."

Grab the instructions from here.  And if your keyboard is singing right now,  someone read this before you :)