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.”

It’s not my joke, I only remembered it! However, I can’t remember where I heard it so I’ll post this source here so as not to be accused of plagiarism later. 🙂

Source: Christine Santorini Biser and Bob Julia