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 way to save your marriage
A young husband with an inferiority complex insisted he was just a little pebble on a vast beach.

The marriage counselor, trying to be creative, told him, "If you wish to save your marriage, you'd better be a little boulder."