My (limited) understanding is that the shape is a large factor. Something about sharp edges or points concentrating the electric field induced by the microwaves, leading to a high voltage buildup. So forks are a bad idea, while spoons will likely not result in any sparks.

I once put food covered with alumnium foil (that had ton of creases) to heat up. That was fun show

My aunt did that with baked potatoes in ~1990 and burned down the entire house.