Vaseline

A woman answers the door to a market researcher. "Good morning madam, I'm doing some research for Vaseline. Do you use it at all in your household?"

"Oh yes, all the time. It's very good for cuts, grazes and burns."

"Do you use it for anything else?"

"Like what?"

"Ahem.. err.. well.. during.. ahem.. sex."

"Oh, of course. Yes, we smear it on the bedroom doorknob to keep the kids out."