From an interview with John Mulaney

Interviewer:	I watched HBO’s “Talking Funny” special and at one point Ricky Gervais asks the other comics when they realized they might be funny. I was wondering if you remember a joke from early on that worked well.
Mulaney:	Yes I do. I was at a birthday party when I was 4 and the birthday girls’ mom said, “That’s a sharp shirt,” and I said “Yeah sometimes I use it instead of scissors.” And I said it really deadpan.