@ArunDebray Maybe this is because I am a pessimist by nature, but this joke makes me a bit sad.

(If set of all my friends is empty, that's not a very good feeling.)

@MartinSleziak ah, fair point. OK, here's a happier one:

Pay attention to surjective functions – I think they're onto something!

A mathematician recommended her colleague, an active researcher in algebraic geometry but eager to learn homotopy theory, some notes and leaves him to study. During lunch break she knocks on she opens the door of his study to check, and finds him with his face literally on the paper in front of him. "Are you having trouble?", she inquires. "Not at all!", he says. "The author claimed this diagram was commutative even on the nose, and yup, it still is!"