A man likes sending random stuff to his friends through the mail because he finds it funny. This particular time the man takes some lettuce to the post office to ship to a friend from back home. He tries to package it up but it won't fit unless he cuts it into smaller peices. He cuts it up and stuffs it in a large envelope, however he forgets to write out and attach a shipping label. He doesn't realize his mistake at the time and brings it to the counter to send.
The postal workers says: "You can't send a salad like that, it needs adressing".
A linguistics professor says during a lecture that, "In English, a double negative forms a positive. But in some languages, such as Russian, a double negative is still a negative. However, in no language in the world can a double positive form a negative." But then a voice from the back of the room piped up, "Yeah, right."