Proof Positive?

I know I said there wouldn't really be more jokes, but I hear this one, and it tickled me, so I'm throwing it up here.

A linguistics professor was lecturing to his English class one day. "In English," he said, "a double negative forms a positive. In some languages, though, such as Russian, a double negative is still a negative. However, there is no language wherein a double positive can form a negative."

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