Boiling water is the secret to a hot thermos lunch

The best way to pack a hot meal in a thermos is to preheat the container with boiling water before adding your food. Do the opposite for cold food: pre-chill with ice water.

It only takes a few minutes. Fill the thermos with boiling water, put the lid on, and let it sit for 3–5 minutes while you heat your food. Dump the water, dry it quickly, then fill the thermos to the brim with your piping-hot meal. That step heats the walls, so they store energy instead of stealing it from your food.

For cold meals, use ice water the same way. Or if you’re packing something like yogurt, applesauce, or a smoothie, you can skip the ice bath and just add a little frozen fruit or freeze a drink to keep it cold.

The other rule: fill it as full as you can. Air is the enemy. The less empty space, the longer your food stays at the right temperature. And don’t pop the lid open at 10 a.m. just to check — every opening costs you heat.

A few minutes of prep is the difference between a sad lukewarm lunch and a genuinely great one.