This trick for figuring out how long your freezer was without power is genius.

Facing a major storm is scary enough—packing your bags, securing your home and valuables, and evacuating to safety should be at the forefront of your mind. But when they return, many people are faced with a decision about the food inside their freezers and refrigerators—namely determining whether the food inside it is safe to eat, or if it got too warm for too long.

Should a refrigerator lose power, everything from meat in the freezer to milk in the fridge is at risk, especially if hours turns into days without proper cooling. According to the United States Department of Agriculture, anything left unrefrigerated for over 4 hours at temperatures of 40 degrees or higher is at risk of developing foodborne bacteria that could lead to illness.

We all know that keeping our fridge and freezer closed when power is out is key to maintaining cold temps, but what if you're not home to see how long power has been out?

Luckily for us, one home cook shared her genius trick on Facebook. It’s so simple, and you only need two things: a cup of water and a quarter.

In a Facebook post, Sheila Pulanco Russell says to put a cup of water in your freezer. When it's frozen solid, place a quarter on top of the newly formed ice and leave it there.

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After returning home, you'll have a better picture of how long your freezer (and by extension, your fridge) was not cool. If the quarter is still at the top, your food is safe to eat. But if it has started sinking towards the middle of the cup, you'll know your food has been put at risk of spoiling.

If the quarter made it all the way to the bottom of the cup, it's likely that the refrigerator spent many hours being too warm, and that it's best to toss all food out.

This is not a proven scientific technique, as Russell notes, so listen to your instinct if it feels like something has spoiled. "If you don't feel good about your food, just throw it out," she writes.

This hack has given Russell peace of mind during hurricane season when food is particularly at risk—and it has helped over 400,000 other home cooks, as well, as her post has been shared far and wide since it went live.