"You owe me $5."
Yes, we all know that money and friends don't mix, but also, how could they not? Maybe your friend brushes you off when she covers your part of the girls' night tab and says you can buy her coffee sometime. Did you, though? Did you ever do that coffee? In the grand scheme of a friendship, it's not worth making a fuss over $5 or $10, so she's not going to send you a bill, but her casual generosity is not something to be forgotten. It's an invitation to buy her the biggest, bestest, most ridiculous triple-shot caramel mocha with whipped cream and chocolate sprinkles that has ever existed. Without being reminded.