21. Make someone else start a knock-knock joke:
You: "I know a really cool knock-knock joke, but you must start."
They: "knock-knock"
You: "Who's there?"
They: *really awkwardly looking at you, void of knowledge on how to react*
It is really funny xD
22. Put a wireless mouse USB receiver in your coworkers pc (in the back) and occasionally move the corresponding mouse (that is on your desk of course).
23. When someone (like your parents) live in their house for a looong time and are totally accustomed to how their furnitures are arranged, move all of their furniture a few centimeters, so it is almost not noticable. Result: they will keep walking into the corner of the table, the corner of the sofa, the corner of their bookshelf.
You: "I know a really cool knock-knock joke, but you must start."
They: "knock-knock"
You: "Who's there?"
They: *really awkwardly looking at you, void of knowledge on how to react*
It is really funny xD