An English professor was lecturing to his class one day.
"In English," he said, "a double negative can form a positive. For example, ‘I don’t disagree,’ means ‘I agree.’ And ‘He can’t do nothing’ means ‘He does something.’
“However,” the professor continued, “in some languages, such as Russian, a double negative is still a negative.
“But, there is no language wherein a double positive can form a negative."
A voice from the back of the room piped up, "Yeah, right."