[newfocogi]

I understand the physics of microwaves but haven't taken the time to understand when metal is dangerous — I tend to just always avoid it.

Do you know of any resources that explain what scenarios belong in the "harmless" category?

[cstrahan]

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.

[slt2021]

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

[Henchman21]

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