Frozen water bottles beat ice packs

Freeze a water bottle solid the night before and it will keep your lunch cold longer than most ice packs.

The trick is thermal mass. An ice pack is mostly gel with a thin plastic shell — it thaws fast. A full bottle of water is basically a solid block of ice. It takes a lot of heat to melt it, which means your sandwich stays cold until noon instead of lukewarm by 10 a.m.

A few practical notes. Fill the bottle only about three-quarters full so it has room to expand when it freezes. Freeze it lying on its side or upright, doesn’t matter much. Put it on top of your food, since cold air sinks. And if condensation bothers you, either toss the bottle in a plastic baggie or use one of those insulated lunch bags that keeps the moisture inside.

Bonus: by afternoon you have ice-cold water to drink. That alone beats a squishy blue ice pack.

One warning — don’t use carbonated drinks. The pressure can burst the bottle in your freezer. Plain water works fine and costs nothing.