A linguistics professor was lecturing to her class one day. “In English,” she 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.”
Source: http://www.enginesofmischief.com/makers/evan/humor/doublenegative.html