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."

African Marriage
Young Son: 'Is it true, Dad? I heard that in some parts of Africa a man doesn't know his wife until he marries her?

Dad: 'That happens in all countries, son.'