Here's a funny joke.

Sneak into a co-worker's office, steal their desk calendar, and change it to April 1. Then, when they come into work, you can play a trick on them, and yell "April Fool!" They'll look at the calendar and say "Oh, you got me."

Then you can say "HA, Ha, it's not really April Fools' Day!! You're an IDIOT!!!"

This joke works best on days that are close to April Fools' Day, and is least effective actually on April Fools' Day.