Two old men sittting in an old peoples home. They are sat facing each other.
Frank:[Sniffing the air] Peter, you stink of piss.
Peter: No I don't.
Frank: I'm telling you Pete, you stink of piss.
Peter: And I'm telling you I don't.
Frank: Peter, you always stink of piss. The nurses around here call you Pete Pissy Pants Parfitt. Believe me you stink of piss alright.
Peter: Your wrong.
Frank: You can't tell. You're too close to it and too used to it.
Peter: And I know for definite that I don't stink of piss.
Frank: How eh? How do you know you old fart?
Peter: Because I've just shit myself.