Take the seal out every time

Mold in a lunch box is almost always living in the silicone seal, and the fix is to remove that seal and dry everything completely before closing it back up.

I know it’s annoying. The seal is the most fiddly part of the whole thing and it feels like it should be fine to just leave it in there. But that’s exactly why it molds. Food crumbs and moisture get trapped between the seal and the lid, and the lid seals it all in a nice little greenhouse for mold. If you skip the seal, you’re basically wiping down the box while leaving the problem behind.

Run the seal under hot soapy water (or toss it in the dishwasher if it’s removable, which it should be). Then — this is the important part — dry it and the lid separately before snapping it back in. A few minutes on a drying rack is enough. The goal is zero moisture trapped in there when you close it.

If mold is already growing, soak the seal and the lid groove in white vinegar or a baking soda paste overnight, scrub with a toothbrush, rinse, and dry. Then change your habit going forward. Store the box with the lid cracked or the seal popped out between uses, and it’ll stop coming back.

Future you doesn’t want to scrub black spots out of a gasket at 6 a.m.