A linguistics professor was lecturing to his class one day about the
fact that in many languages, such as English, a double negative forms
a positive, while in other languages, such as Russian, a double negative
is still a negative. However, he pointed out, in no language can
a double positive form a negative.

A bored voice from the back of the room responded, Yeah, yeah….

[Ed: Reportedly a true incident, accounted in the New Yorker Magazine. ]