Freeze a water bottle.

Freeze a water bottle and you’ve got a DIY ice pack that works every bit as well as the real thing.

It’s the oldest trick in the book for a reason. A full bottle of water takes a long time to thaw in an insulated bag, so it keeps everything around it cold for hours. And unlike a real ice pack, when it melts you get a bottle of ice-cold water to drink at the end of your shift. That’s a win-win.

If you have a decent insulated lunch bag, that’s half the battle. Put the frozen bottle on top of the food, not underneath—cold air sinks. Also, stuff any empty space in the bag with a rolled-up towel or crumpled paper. Less air moving around means less heat getting in. Keep the whole bag out of direct sun or off a hot metal shelf if you can.

No water bottle? Freeze a juice box, a small container of yogurt, or even a sock filled with dry rice. Anything that’s mostly water and sealed will do the job.

This is one of those rare cases where the hack is genuinely just as good as the product you’d buy.