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 Night at the Asylum
Late one night at the insane asylum one inmate shouted, "I am Napoleon!"
Another one said, "How do you know?"

First inmate answers, "Winston Churchill told me!"

Just then, a voice from another room shouted, "I did NOT!"