Your lunch bag needs to be a cooler, not a tote.

Keeping a beach lunch cold without a cooler comes down to two things: a real insulated bag and something frozen inside it.

First, don’t grab a thin insulated grocery bag. You want a lunch bag or soft-side cooler with thick insulation all the way around, a tight zipper, and a wipeable interior. It doesn’t need to cost a hundred bucks — anything that’s actually made to hold ice will do. If you can feel the cold through the side when you hold it, it’s too thin.

Second, add something frozen. Ice packs are fine, but frozen water bottles work just as well and double as drinking water later. You can also freeze a yogurt cup, grapes, or a juice box and use those as edible ice packs. Put the frozen thing on top of the food, because cold air sinks and will flow down over the rest of the bag.

Then keep the bag out of the sun. Put it under a towel, in the shade of your umbrella, or half-buried in the sand. Open it only when you need to. That’s the whole trick: cold starts cold, stays cold, and you don’t give it extra heat.

In real beach heat this is good for a few hours, not all day — so plan to eat before the ice melts.