Can you microwave stainless steel?

A stainless steel bowl or travel mug acts as a shield: microwaves reflect off it, so the food inside barely heats while energy bounces around the cavity. The rim and any thin lip concentrate charge and spark against the oven wall. Insulated stainless flasks are worse again, because the vacuum wall means the outer surface tells you nothing about how hot the contents are. There is no version of this that works, including "just for ten seconds".

What goes wrong

• Arcing at the rim damages the cavity and the waveguide cover
• Reflected energy with nothing absorbing it stresses the magnetron
• Contents heat unevenly or not at all, which people mistake for cold food and re-run

What to do instead

• Pour the drink into a ceramic mug
• Heat soup in a glass or ceramic bowl and transfer it to the flask after

Other "can you microwave" questions

• Can you microwave styrofoam? It depends — Only if the container is labeled microwave-safe, and most takeout foam is not.
• Can you microwave aluminum foil? No — No — foil reflects microwaves and the edges arc, which can start a fire.
• Can you microwave plastic containers? It depends — Only ones labeled microwave-safe — cold-storage tubs are not among them.
• Can you microwave eggs? It depends — Beaten or pricked eggs, yes. A whole egg in its shell will explode.
• Can you microwave Tupperware? It depends — Yes, if that specific piece carries the microwave-safe symbol on its base.
• Can you microwave paper plates? It depends — Plain white paper plates are fine; anything with metallic trim or a foam layer is not.
• Can you microwave metal? No — No — metal reflects microwaves, arcs at edges, and can damage the oven or start a fire.
• Can you microwave breast milk? No — No — microwaves create hot spots that scald a baby even when the bottle feels cool.
• Can you microwave takeout containers? It depends — Only if the container is marked microwave-safe — most are not, and metal-handled ones never are.
• Can you microwave cardboard? It depends — Plain dry cardboard is usually fine briefly; anything with glue, staples, wax or foil is not.
• Can you microwave glass? It depends — Yes for ordinary heatproof glass; no for leaded crystal, and never straight from the freezer.
• Can you microwave water? It depends — Yes, but very clean water can superheat and erupt when you move the cup.