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 walking economy
This guy is walking with his friend, who happens to be a psychologist. He says to this friend, "I'm a walking economy."

The friend asks, "How so?"

"My hair line is in recession, my stomach is a victim of inflation, and both of these together are putting me into a deep depression!"