There is a piece of foil in the bottom of the microwave popcorn bag and the metal ring stays when you rip the top off of a microwaveable cup of Mac n cheese. It is far from an absolute prohibition. You just need enough mass to absorb most of the energy. OTOH guys who work on them say popcorn may be the hardest thing people do to their microwave. I knew a guy who said that was all he did with his microwave.