Your lunch box is probably fine in the dishwasher. Your thermos isn't.

Most lunch boxes are dishwasher-safe, but anything vacuum-insulated, melamine, or made of thin plastic should stay out.

The rule is boring: flip it over and look for the dishwasher-safe symbol. Polypropylene plastic (the stiff, slightly waxy kind used in most bento boxes) handles the machine fine. Put it on the top rack and skip the heated dry cycle if you want it to last. Thin, flimsy plastic — the kind that bends when you pinch it — will warp and crack eventually.

The real danger zone is anything with insulation. Vacuum-sealed thermoses and insulated food jars can lose their seal and stop insulating if they take repeated hot washes. Same with the silicone gasket under the lid of a leakproof container: it gets beaten up over time and eventually stops sealing. Run those on the top rack or just hand-wash them — it takes twenty seconds.

Melamine and bamboo fiber lunch trays are not dishwasher-safe, even if they look like plastic. Aluminum lunch boxes will come out dull and pitted, because dishwasher detergent attacks metal. Glass and single-wall stainless steel containers are fine, though stainless picks up water spots.

Hand-washing the thermos beats re-buying it every year.