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 serious condition
A well respected surgeon was relaxing on his sofa one evening just after arriving home from work. As he was tuning into the evening news, the phone rang. The doctor calmly answered it and heard the familiar voice of a colleague on the other end of the line."We need a fourth for poker," said the friend."I'll be right over," whispered the doctor. As he was putting on his coat, his wife asked, "Is it serious?""Oh yes, quite serious," said the doctor gravely."In fact, three doctors are there already!"