Linguistic joke

A linguistics professor was lecturing the class.
“In English,” he explained, “a double negative forms a positive. In some languages, such as Russian, a double negative is still a negative.”
“However,” the professor continued, “there is no language wherein a double positive can form a negative.”
Immediately, a voice from the back of the room piped up: “Yeah… right…”
(Posted on the LinkedIn Anthropology and Linguistics Group)