Help with Question to The Mousetrap: The Agatha Christie play

The Agatha Christie play The Mousetrap has not only the longest initial run of any play in history (it’s been running continuously for nearly 65 years, mostly in the very same building in London), but also an ending which is traditionally kept secret, the audience asked after every performance not to reveal it except to others already in the know.
My question is: why is the play called The Mousetrap?