A fellow was going on a tour of a factory that produces various latex products. At the first stop, he is shown the machine that manufactures baby-bottle nipples. The machine made a loud 'hiss-pop!' noise.

"The hiss is the rubber being injected into the mold," explained the guide. "The popping sound is a needle poking a hole in the end of the nipple."

Later, the tour reached the part of the factory where condoms were manufactured. The machine made a noise: "Hiss. Hiss. Hiss. Hiss-pop!"

"Wait a minute!" said the man taking the tour. "I understand what the 'hiss, hiss' is, but what's that 'pop!' every so often?"

"Oh, it's rather like the baby-bottle nipple machine," said the guide, "but here the needle pokes a hole in every fourth condom."

"Well, that can't be good for the condoms!"

"No," the guide said, "but it's great for the baby-bottle nipple business!"