A linguistics professor was lecturing to his 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."
(I would love to credit this but it was something I collected a bazillion years ago from an email that was making the rounds.)