Wash it daily, then let it breathe.

The best way to keep a lunch box mold-free is to wash it with hot soapy water after every use, dry it completely, and leave the lid open overnight.

Mold needs two things: food residue and trapped moisture. Your lunch box gives it both if you wipe it out, snap it shut, and toss it in your bag. That’s basically a petri dish with a handle. Hot soapy water cuts the grease and food bits. Drying matters just as much — even a few drops in the corner can feed mold by morning.

You don’t need special cleaning products. Dish soap works fine. If you have a lingering odor already, wash it, dry it, then leave a small open box of baking soda inside for a few hours. Vinegar works too, but nobody wants a vinegar lunch box. Rinse it well.

The real trick is habit: clean it when you get home, not right before packing in the morning. The extra hour or two of open-air drying is what actually prevents the problem.

Closed damp lunch boxes are where mold goes to party.