Linguistic humor, Double positive

Source: Brought to my attention by Christine Santorini Biser and Bob Julia. See also Sidney Morgenbesser.

An MIT linguistics professor was lecturing his class the other day. "In English," he said, "a double negative forms a positive. However, in some languages, such as Russian, a double negative remains a negative. But there isn't a single language, not one, in which a double positive can express a negative."

A voice from the back of the room piped up, "Yeah, right."