I saw this somewhere a week or so ago. I don't remember where, so I can't credit that site. But when I did a Google search, it came up with this location.
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."