I don’t know who wrote this but I like it:

A linguistics professor was lecturing to his English 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 where in a double positive can form a negative.”

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

It sounds like “yeah” and “right” were more complex than I thought …