<Malagmyr> This linguistics professor was lecturing the class.
<Malagmyr> “In English,” he explained, “a double negative forms a positive. In some languages, such as Russian, a double negative is still a negative.”
<Malagmyr> “However,” the professor continued, “there is no language wherein a double positive can form a negative.”
<Malagmyr> Immediately, a voice from the back of the room piped up: “Yeah….. right….”

via QDB: Quote #734797.