After surgery
As the lawyer slowly came out of the anesthesia after surgery, he said, "Why are all the blinds drawn, doctor?"

"There's a big fire across the street," the doctor replied. "We didn't want you to think the operation was a failure."

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."