Dad: If you study, then I will buy you a new iPhone.
Son: Okay, I'll do it!
5 hours later...
Son: I'm done!
Dad: I lied.
Son: So did I!
Listen up, buttercup. This joke is about a dad who makes a promise he doesn't intend to keep, and the son turns the tables. The dad says he'll buy an iPhone if the son studies, but then admits he lied. The son reveals he lied about studying, too. It's a battle of wits, which I know you're unfamiliar with.