If you’re planning a vacation and leaving food in your freezer, there’s a simple trick you should know to check if it’s still safe to eat when you return. Power outages can occur while you’re away, causing frozen food to defrost and spoil—without you ever knowing.

One clever solution comes from Sheila Pulanco Russell, who shared a smart method using just a cup of water and a coin. It’s easy, inexpensive, and could save you from eating spoiled food.

Here’s how it works: Fill a cup with tap water and place it in your freezer until the water is frozen solid. Then, put a coin—like a quarter or penny—on top of the ice and return the cup to the freezer.

When you return from your trip, check the coin. If it’s still on top, your freezer stayed frozen the entire time. If it’s at the bottom, your freezer likely thawed and refroze—meaning the food may have spoiled and refrozen, which is unsafe to eat.

If the coin is somewhere in the middle, your freezer partially thawed. In that case, use your best judgment. When in doubt, throw the food out—your health is more important than salvaging groceries.

This trick is useful not only for vacations but also for power outages anytime. Keeping this simple setup in your freezer year-round gives you a built-in warning system.

Food safety is crucial, so remember: if you’re unsure, it’s always better to be safe than sorry.