“I bought a doughnut and they gave me a receipt for the doughtnut... I don't need a receipt for the doughnut. I give you money and you give me the doughnut, end of transaction. We don't need to bring ink and paper into this. I can't imagine a scenario that I would have to prove that I bought a doughnut. To some skeptical friend, 'Don't even act like I didn't get that doughnut, I've got the documentation right here... It's in my file at home. ...Under "D".'
Really funny when you think about how this joke aged in this era of rampant privacy abuse.