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."

An accountant is having a hard time sleeping...
An accountant is having a hard time sleeping and goes to see his doctor."Doctor, I just can't get to sleep at night." "Have you tried counting sheep?""That's the problem - I make a mistake and then spend three hours trying to find it."