Frozen water bottles make the best ice packs.

Frozen water bottles are the best thing you can put in a hiking lunch bag — they keep your food cold for hours and turn into drinking water by the time you stop.

The system is simple: an insulated bag, something frozen, and food that doesn’t mind being a little warm. A 1-liter frozen bottle is both ice pack and water supply. Freeze two if it’s a long day — one stays on top of the food, the other gets drunk by lunch.

Pack the bag as full as you can. A packed lunch bag holds cold much better than a half-empty one because there’s less warm air sloshing around. Fill gaps with another frozen bottle, an apple, even a rolled-up towel. And keep the bag in the shade — inside your main pack is better than clipped to the outside in direct sun.

Then pick food that tolerates being out of the fridge for five hours: hard cheese, cured meat, nut butter sandwiches, hummus, apples, oranges, trail mix. Avoid mayo and anything with raw egg or soft dairy that’s sat out all morning. If you’re carrying raw meat or cooked chicken, treat it like a real cooler — more ice, and pack it separate.

A hangry hiker is a liability, so spend the extra two minutes with the freezer.