Grammar joke 

The kind of thing that makes an English teacher smile. And even if it doesn't make you smile it will at least remind you that there are exceptions to every rule.

A linguistics professor was lecturing to his class one day:
"In English," he said, "A double negative forms a positive. In some languages, 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."