But a prank is always at the target's expense. You are hurting, at the very least, their ego and/or pride. Or you are startling them, or surprising them (unpleasantly).
That's the difference between a joke and a prank. A joke is told. A prank is done to someone.
This is technically correct, but traditionally you're only supposed to execute pranks against friends that you already know would appreciate this kind of humor. If that's what you're doing, it's not usually a negative thing for anyone involved.
Put pocketing occurs to me as a simple prank that doesn't hurt anyone. It's all about making someone part of the joke, rather than the butt of the joke.
That's the difference between a joke and a prank. A joke is told. A prank is done to someone.