A college professor of logic was attempting to teach his young son the principles of clear thinking and the necessity for defining all terms. He pointed to a wall clock which had just struck.

Now, if I were to take a hammer and smash the clock, he queried, could I be arrested for killing time?

The lad hesitated a moment. No, he said, itd be self-defence.

The professor frowned. How do you figure that out ?

Because, answered the boy, the clock struck first.