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."

747 Full of Lawyers
Q: Hear about the terrorist that hijacked a 747 full of lawyers?

A: He threatened to release one every hour if his demands weren't met.