It looks like a forgotten mug: a cup of ice in the freezer with a coin sitting on top. Tom’s Guide recommends setting it up before a winter storm, and many people do it before a trip. That small coin answers a question you really don’t want to guess at after a power outage, but it has a weakness few people know about.
In short
- Freeze a cup of water, then set a coin on top of the ice.
- If the coin has sunk, the freezer warmed up long enough for the ice to melt while you were away.
- Food safety depends on temperature: a thermometer is more reliable than a coin.
- A full freezer holds its temperature for about 48 hours if the door stays closed.
What the coin is for
The trick is simple. As Tom’s Guide explains, fill a cup with water, freeze it overnight, place a quarter on the ice and put the cup back in the freezer. Then you just check where the coin is:
- Still on top: the ice didn’t melt.
- In the middle: the ice partly thawed, then refroze. Check each item.
- At the bottom: the freezer fully defrosted at some point, and the food should be thrown out.
The point is that a power cut can happen while you’re asleep or away and leave no trace. When the electricity comes back, the freezer refreezes everything, and the food looks normal again. The coin keeps a record of what happened.
Why it matters after an outage
Thawed and refrozen food can’t be judged by its look or smell, and FoodSafety.gov is categorical: “Never taste food to determine its safety.” The federal site also sets the rules that matter: food “may be safely refrozen if it still contains ice crystals or is at 40°F (4°C) or below,” and a full freezer holds a safe temperature for about 48 hours, or 24 hours if it’s half full, as long as the door stays closed.
Other clues to check
The coin is only one signal. Tom’s Guide points out that water pooled inside packaging is a sign that food has thawed and refrozen, which is generally unsafe, especially for meat, poultry and seafood. The same article recommends checking with an appliance thermometer, or a Wi-Fi fridge thermometer that sends an alert when the temperature rises. Keeping the freezer full also helps: it holds the cold twice as long as a half-empty one.
The limit
The coin has a known flaw. NC State Extension, in an article titled Coin in the Cup: RISKY, points out that ice floats: if it melts and refreezes without completely turning to water, the coin can stay on top. The trick “would give a false sense of food safety,” the extension warns, and it never tells you the actual temperature. Food safety experts quoted there suggest a temperature logger or color-changing temperature strips instead. The coin is a free first clue, not proof. In doubt, use an appliance thermometer, and when you’re still unsure, throw it out.
Storm season is also the time to know which appliance to unplug during a storm, and the one kitchen appliance firefighters say to unplug before bed.
A cup, a quarter and a quick look. That’s the trick.
