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 man with a glass eye is here to see you
Nurse: Doctor, there is a man in the waiting room with a glass eye named Brown.

Doctor: What does he call his other eye?