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."

Give a man a fish
Give a man a fish and he will eat for a day. Teach him how to fish and he will sit in a boat and drink beer all day!