Quote from Rachel in The One with the Boobies
Ross: All right, all right. We're all adults here. There's only one way to resolve this. Since you saw her boobies, I think you're going to have to show her your pee-pee. Chandler: You know, I don't see that happening. Rachel: Come on, he's right. You know, tit for tat. Chandler: Well, I'm not showing you my tat.