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."

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."