Jan Šíla

“Once, sitting in my car in the parking lot outside Building 5 during a prolonged downpour after a long lunch off premises, Mark, Larry, and I pondered the famous two-condom combinatorial problem that spread through the Center: Two (heterosexual) couples decide to have group sex with each other in all possible male-female combinations. They have only two condoms, and everyone is scared of catching some venereal disease. How can they manage four couplings with only two condoms? The first man puts on two condoms, one over the other, and then sleeps with the first woman. Only the outer surface of the outer condom and the inner surface of the inner one has had contact with any potentially infectious surface. The man removes the outer condom and sleeps with the second woman. The second man then dons the removed outer condom whose inner surface has until now had no contact with anyone’s skin, and sleeps with the first woman, whose only contact has thus far been with the outside of the same condom. Finally, the second man dons the second condom over the one he is already wearing, and sleeps with the second woman, who again only experiences a condom she has already touched. It was impossible to resist the temptation to generalize to N couples.”
― Emanuel Derman, My Life as a Quant: Reflections on Physics and Finance