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."

Can I play the piano once these are off?
A doctor has come to see one of his patients in a hospital. The patient has had major surgery to both of his hands.

"Doctor," says the man excitedly and dramatically holds up his heavily bandaged hands. "Will I be able to play the piano when these bandages come off?"

"I don't see why not," replies the doctor.

"That's funny," says the man. "I wasn't able to play it before."