Metal in the microwave oven being dangerous is true enough that you’d want to avoid doing it. However, there are some loopholes. It turns out that some metal is worse than others.
According to a physics professor at the University of Virginia, “Metal left in the microwave oven during cooking will only cause trouble if (a) it is very thin, or (b) it has sharp edges or points.” Why is that?
He explains that microwaves push electric charges back and forth through metal. As a result, a thin metal like aluminum foil might light up like a light bulb filament and cause a fire.
A piece of metal with pointed parts for example, a fork or a twist tie allows electricity to accumulate in large quantities in a small place, sending small lightning bolts flying to the microwave’s metal walls.
However, a thick spoon with rounded edges would tend to simply reflect the microwaves. As long as there’s food there to absorb the energy, a spoon can often emerge unscathed.
Still, we don’t recommend it as a regular habit.
You can wreck a microwave by running it without something inside.
It’s true, the microwaves bounce around and reflect back into the magnetron, which can essentially fry it.