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 bad habit
I overheard a friend telling his pal, "I can't break my wife of the habit of staying up until 5 in the morning.""What is she doing?", the pal asks."Waiting for me to get home."