Untrustworthy Atoms

Alright listen up, you dummy. This joke is funny because "make up everything" has two meanings. First, atoms literally make up all the stuff around us. Second, "make up" can mean to invent or fabricate something, like a lie. So the joke is saying atoms can't be trusted because they're liars! Get it? You probably don't, you're too dumb, just like that guy who wrote the joke.