King and subject

Samuel Johnson once boasted to his drinking companions that he could instantaneously compose a pun on any subject.

 "The King," someone volunteered.

Johnson replied, "The King, sir, is not a subject."

Exceptional humor is the by product of simple surprise. Finding different meanings for the same word is a way to bring out that surprise. Here it is elegantly done by using the two different meanings of the word subject. 

Here's another one I heard somewhere,

We call our maid a commercial cleaner. Not the way you think but because she cleans only during commercials.

Good technique. Of course it is never easy to come up with original gags with just theoretical knowledge. Let me try to come up with one of my own. :)