'hiss-pop'

A guy is 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 makes a loud "hiss-pop" noise.

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

Later, the tour reaches the part of the factory where condoms are manufactured. The machine makes a "Hiss. Hiss. Hiss. Hiss-pop" noise.

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

"Oh, it's just the same as in the baby-bottle nipple machine," says the guide. "It pokes a hole in every fourth condom."

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

"Yeah, but it's great for the baby-bottle nipple business!"