One day, a market researcher was knocking on doors in a street. He reached the final house in the row and was welcomed by a busy looking housewife.

“Hello,” he started. “I’m doing some research for a petroleum jelly manufacturer. Have you ever used the product?”

“Sure,” the busy woman answered. “My husband and I use it when we’re intimate.”

The researcher was taken aback and stuttered: “Oh, er, I admire you for your honesty. Can you tell me exactly how you use it?”

“Yeah,” she replied. “We put it on the doorknob so the kids can’t get in.”