Most lunch boxes are dishwasher-safe. The lid is the problem.

Most lunch boxes can go in the dishwasher, but the lids, seals, and insulated models are where things go wrong.

If it’s a solid plastic body with a “dishwasher safe” mark on the bottom, it’ll be fine on the top rack. Glass is fine too, just heavy and noisy. The trouble starts with the lid. Many lids have silicone gaskets that trap water, and the plastic can warp in the heat, ruining the seal. That’s how you end up with a lunch box that won’t snap shut.

Insulated stainless steel containers—thermoses, vacuum bento boxes—should stay out of the dishwasher entirely. The high heat can damage the vacuum seal and wreck the insulation. Same goes for any lid with rubber parts.

When in doubt, hand-wash the lid and any rubber pieces. The body can take the machine. And if the box doesn’t say dishwasher-safe, trust the label, not your hope.

Future You deserves a lunch box that still closes.