I remember when domestic microwaves first came out and the press told everyone that they were really dangerous if you turned them on when empty and would blow up so everyone used to keep a little bowl of water inside just in case the thing should ever accidentally spring to life. people are funny
Alternatively, you could use a little kitten.
good idea though it would need to be a dead one as a live one would just walk out. you should never shut the door as this would increase the chance of accidental oven activation, it was all over the press at the time