Condensation happens before you close the lid.

Condensation dripping on sandwiches is mostly caused by warm food and trapped air, so the fix is cooling things down and putting a barrier between the bread and the box.

The biggest culprit is warm food. If you pack a sandwich next to a still-steaming container of soup or leftovers, that moisture has to land somewhere — usually the lid, then on your lunch. Let hot food cool to room temperature before you seal the box. That alone fixes most of the problem.

Next, don’t trust the box to protect the sandwich. Wrap it in wax paper or put it in a small container. Even a paper towel wrapped around it will soak up moisture and keep the bread from turning soggy. If you’re using a frozen ice pack, keep it from touching the sandwich directly — condensation collects on the pack surface and drips down.

Finally, leave less air inside. Condensation forms when humid air hits a cool surface, so a mostly empty box gives moisture more room to form. Fill the empty space with fruit or a snack, and dry the inside of the box before you close it.

Your sandwich doesn’t need a sauna.