I bought a doughnut and they gave me a receipt for the...

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".'