Put the ice pack on top, not the bottom

Put the ice pack on top of your food, not under it, and make sure your lunchbox is actually insulated. That’s the whole trick.

Cold air sinks. If the ice pack is on the bottom, the cold air just sits there at the bottom of the bag and your sandwich on top stays lukewarm. Flip it — ice pack on top, food below — and the cold air rolls down over everything. It’s not magic, it’s just physics working in your favor.

A few other things that actually matter:

• Freeze the pack flat, so it has more surface area touching your food. A thick lump of ice in the middle of the bag does less work than a flat slab.
• Keep the lunchbox closed. Every time