The rubber gasket is almost always the culprit

The rubber gasket is almost always the culprit when a thermos leaks in a lunch box. Check that first before buying a new one.

Pop the lid open and look at the rubber ring inside. It might be twisted, pinched, or sitting crooked. Fixing that usually fixes the leak. If the gasket looks cracked, stretched, or feels hard, replace it—most thermos brands sell spare gaskets.

Tighten the lid firmly, but don’t crank it like a jar of pickles. Over-tightening can actually squish the gasket out of shape and create a gap. Also, give hot liquids a little headspace—leave an inch or so of air. Steam pressure builds up and forces liquid past the seal, especially if the thermos rolls on its side.

Still worried? The cheap insurance is a sandwich bag. Slip the thermos in before it goes in the bag. It’s not elegant, but it turns a disaster into a damp napkin. And do a quick test: fill it with water, close it, turn it upside down, shake it. If it doesn’t leak that way, it shouldn’t leak in a lunch box.

Most leaks are beatable with a finger and a rubber ring. That’s a better first move than tossing it.