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,” he pointed out, “there is no language wherein a double positive can form a negative.”

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

Bonus Pun:

They told me I had type-A blood, but it was a Type-O.