The mice were much smarter than this trap.  When I saw it with the door shut, I assumed I had caught my mouse, and drove miles away to release it, trying to be nice and humane.  When I arrived and opened the door, nothing happened.  There was no mouse.  There was only evidence of a mouse, as he had chewed all over the trap, and chewed off part of the door hinge mechanism, so that it no longer was functional.  The design was faulty as well as the opaque black color - you can't look inside to see if someone is in there!  I had to return it and bought a different one, see-through plastic and with a better trap mechanism.  I successfully caught two mice with the second trap.  Don't waste your money on this one, it is ineffective even though it is cheap.