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 walked into
A man walked into the local Chamber of Commerce of a small town, obviously desperate. Seeing a man at the counter, the stranger asks, ''Is there a criminal attorney in town?''

To which the man behind the counter immediately quipped, ''Yes, but we can't prove it yet!''