Jokes / Double positive

Jokes, School Jokes,

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 A case for the Third Universal Cardinal Rule of Thumb: Never
 be absolute, unless absolutely necessary:
 

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."