This is from Saturday Night Live for 2/08/86 and it's the bit that made me really love Penn & Teller. I'd seen them before — in person, in fact — but this routine sealed the deal. It's also, interestingly, a bit they soon retired because they got too old to do it. Not long ago, it was resurrected for their Penn & Teller Fool Us series but with two young women standing (or hanging) in for them.
Make sure you watch it 'til the end…and make sure you realize that it's a perfect routine for Saturday Night Live. It could never have the same impact on any pre-recorded show…