As Daffy Duck once said, "It is to laugh."
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...."