A lecture about English
A linguistics professor was lecturing to his English class one day. "In English," he said, "A double negative forms a positive. In some languages, though, such as Russian, a double negative is still a negative. However, there is no language wherein a double positive can form a negative."

A voice from the back of the room piped up, "Yeah, right."

A precise location
Attorney to witness: "And where was the location of the accident?"Witness: "Approximately milepost 499."Attorney: "And where is milepost 499?"Witness: "About halfway between milepost 498 and milepost 500."