Before leaving for vacation, there’s a surprisingly simple trick that could help you figure out whether your freezer lost power while you were away. All you need is a glass of water and a coin. Freeze the water solid, place the coin on top of the ice, and leave it in the freezer before you go.

When you return, check the coin. If it’s still sitting on top, the ice likely remained frozen. But if the coin has sunk into the ice, it suggests the freezer warmed enough for the ice to partially melt before freezing again. That can be an important warning sign that a power outage occurred while you were gone.

A full freezer can generally keep food frozen for about 48 hours without power, while a half-full freezer may stay cold for around 24 hours, provided the door remains closed. If food has thawed but stayed at refrigerator temperature (around 4°C/40°F or below), some items may be safely refrozen, although their quality can suffer.

However, if perishable foods such as meat, seafood or dairy have completely thawed and been at room temperature, it’s safest to throw them away rather than risk food poisoning. So before your next vacation, try the frozen-coin trick—it takes almost no effort and could give you a useful clue about what happened inside your freezer.