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Believe it or not, you don’t need a fridge to keep food cold. Just gather some ceramic pots, sand, and water, and you’ve got a portable, non-electric mini-fridge with a time-tested design. After all, people were preserving food for thousands of years before you had to keep that leftover takeout from stinking up your college dorm.

In some parts of the world, this clay pot cooler is called a zeer, and its sustainable, inexpensive design is far from new. People in the Middle East and Africa have long used similar contraptions to keep food from spoiling in hot, dry climates.

“It’s just amazing,” says Paul Smith Lomas, CEO of Practical Action, a U.K. charity that helps people in Latin America, East and Southern Africa, and South Asia find solutions to daily challenges, including food preservation. “We like to find ingenious ideas that can help people fix their own problems.”

Here’s how to make one:

  1. Get two unglazed ceramic pots—one that will fit inside the other—plus some sand and water.
  2. Fill the bottom of the larger pot with a couple inches of sand.
  3. Put the smaller pot in the larger one.
  4. Fill the space between the pots with sand.
  5. Pour water into the sand.
  6. Cover the pots with a ceramic lid or wet cloth.

Done. You’re ready to store food inside. Just remember to add water to the sand every day, because zeer pots use evaporation to cool food.

As water evaporates through the clay, it releases energy into the air and cools the space inside the pot. It’s like splashing water on your face on a hot day; the water evaporates off your skin, cooling it in the process. Refrigerator coolant actually works in a similar way, using evaporation to draw heat out of the fridge itself. That’s why the back of your Frigidaire is so warm.

These pot-in-pot coolers are useful in places that don’t have power grids, but they’re also great for people who don’t have fridges, need more space, or want to cut their energy bills. On that last point: they’re also much better for the environment since they don’t require any sort of fuel, much less oil or gas.

It’s best to keep zeer pots in the shade, since the sun will warm them up, but you can also put them in a breezy area—wind makes the water evaporate faster, which cools the food more quickly. They’re most effective in arid climates, because water evaporates more when there’s less of it in the air. So, these pots will likely work better in Arizona than Florida.

They work well in Sudan, where Practical Action has introduced the zeer pot to many in need of refrigeration.

Food security is a huge problem in the Northeast African country, and the homemade coolers can make food last 10 times longer, Lomas says.

“Someone told me they once made ice out of a zeer pot,” he says, laughing.

He didn’t believe the guy, but he does find the pots incredibly effective. According to Practical Action’s website, one woman, Hawa Abbas, used to watch half her okra, tomato, and carrot crops spoil. After discovering zeer pots, that changed.

“They keep our vegetables fresh for three to four weeks, depending on the type of crop. They are very good in a hot climate such as ours where fruit and vegetables get spoiled in one day,” she told Practical Action. “Since I learned how to make zeer pots, our life has been so much better.”

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