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A penny on ice can show that the water in its cup melted and refroze enough for the coin to move. It cannot tell you whether the food in your freezer stayed cold enough to be safe. A coin that remains on top is no reassurance either: the cup can partially or substantially melt and refreeze without the coin sinking, according to North Carolina Cooperative Extension. Treat the trick as, at most, a crude thaw indicator—not a food-safety test.
What the coin can—and cannot—tell you
The trick uses a cup of frozen water as an improvised indicator. If the ice melts enough, the coin may sink; if the water then refreezes, the coin may be trapped lower in the ice. That suggests the cup thawed and refroze. It does not establish the temperature history of food elsewhere in the freezer or how long any food was too warm.
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A cup is a small body of water, not a calibrated sensor. Food packages and locations in a freezer can warm at different rates. A coin still near the top does not prove that the food stayed frozen; a coin that has sunk does not, by itself, prove that all food is unsafe. North Carolina Cooperative Extension warns that a cup can melt substantially and refreeze while the coin stays on top, creating a false sense of safety.
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What to do after a power outage
USDA and FDA guidance makes the decision item by item: frozen food that still contains ice crystals, or is at 40 °F or below, can be safely refrozen or cooked. Use a food thermometer where appropriate; do not rely on a coin, appearance, or odor to determine safety, and never taste food to test it. For food that does not meet the safe-to-refreeze criteria, follow the official food-specific advice and discard it as directed.
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Keep the freezer door closed during an outage. USDA estimates that a closed, full freezer will hold its temperature for about 48 hours and a half-full freezer for about 24 hours. These are general estimates, not guarantees for a particular appliance or its contents. They are not times you can infer from where a coin ends up.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Prepare with temperature readings, not a coin
Keep an appliance thermometer in the freezer, between frozen packages, before an outage occurs. USDA recommends a freezer temperature of 0 °F or below. Its appliance-thermometer instructions say to wait 5 to 8 hours before checking a reading in a freezer and give 0 to 2 °F as the adjustment range for that check. A thermometer reading is useful evidence, but it is not necessarily a log of the freezer’s temperature throughout an outage; the food’s condition still matters.
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- Measuring range:-20~80℉
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If you did not have an appliance thermometer in the freezer before the outage, assess the food packages individually and use a food thermometer as appropriate. USDA’s emergency food guidance and the FDA’s power-outage guidance explain how to handle food after a loss of power. USDA also provides appliance thermometer advice and freezing and food-safety information.
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- Temperature range: -40°to 80°F or -40°to 27°C
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- Measuring range:-20~80℉
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- Please give the thermometer enough reaction time
How the tools differ
| Tool | What it tells you | What decision it supports | Main limitation |
|---|---|---|---|
| Coin in a cup of ice | Whether water in that cup may have melted enough for the coin to move | No reliable food-safety decision | Gives neither food temperature nor duration; the coin can stay on top after substantial melting and refreezing. |
| Freezer appliance thermometer | The freezer temperature when you read it | Monitoring freezer operation; one piece of evidence after an outage | A reading is not necessarily a record of the full temperature history. |
| Food thermometer and package-by-package check | Temperature of an individual food and whether it still has ice crystals | Applying USDA and FDA criteria to decide whether frozen food can be refrozen or cooked | Food must be assessed individually; do not substitute appearance, odor, or tasting for the agency criteria. |
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