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 couple came upon a wishing well...
A couple came upon a wishing well. The husband leaned over, made a wish and threw in a penny. The wife decided to make a wish, too. But she leaned over too much, fell into the well, and drowned. The husband was stunned for a while but then smiled and said, "It really works!"