Ice packs go on top.

Put the ice pack on top of your food, not underneath it, because cold air sinks.

Most people just toss the ice pack at the bottom of the bag. That works, but it’s not optimal — the cold air stays low while the upper half of your lunch box slowly warms up. Put the ice pack on top of your food containers, and the cold air will roll down across everything inside. One ice pack on top beats one on the bottom.

If you have two ice packs, use both: one on the bottom as a cold base, one on top to drop cold downward. That gives you cold from both directions. But if you’re working with just one, top wins.

Also, pack the box full. Empty air is your enemy — it warms up easily and makes the whole bag less cold. Use an already-chilled drink bottle or fruit cup as filler. And start with food that’s already cold; an ice pack isn’t strong enough to chill warm food fast enough.

Your sandwich doesn’t need to be frozen. It just needs to stay cold.