If you’re also a fan of zip-top bags, there’s one thing you might not be using them for, but definitely should be. In fact, we think it’s the very first thing you should do when you pick up a new box: Fill a bag (or two) about 2/3 of the way with water, freeze it, and you’ve got a giant ice cube, which can come in handy in a number of situations.

For example, you’re making a big-batch cocktail and want it to stay cool without getting too watered-down. A solid block of ice will chill your beverage for longer and melt more slowly than a bunch of smaller cubes. The extra-large chunk can also be used for your cooler if you’re going to the beach or planning a picnic. It can act as a re-freezable ice pack in a pinch. In the case of a power outage, it will keep perishables cool and is also an emergency stash of water. And a bag of frozen water in a mostly empty freezer will help your appliance run more efficiently.

Convinced? It’s super simple to do.

Make a DIY Ice Pack with Zip-Top Bags

Take your zip-top bag and fill it with water until it is 2/3 of the way full. You can also opt for less water, but don’t overfill it! You want room in the bag for the water to expand as it freezes. Put it in the freezer, ideally sort of nestled between some other things to give it a bit of shape. And that’s it.