How many times have you heard that if you’re leaving your house before a storm, tornado, hurricane, wildfire evacuation—or even a long vacation—that you should freeze a cup of water and put a penny on top? Well, it turns out this trick doesn’t work and could actually hide the fact that your power went out.
What is the penny in the freezer trick?
According to legend (and to an old Lifehacker post, which probably helped popularize it, unfortunately), the idea is that a frozen cup of water with a penny on top will tell you whether your freezer lost power and warmed up during a power outage.
The idea is that when you’re away for weeks, your freezer could lose power without you knowing. Your food thaws and maybe begins to spoil, but then the power comes back on and refreezes your now-bad food. If you leave a penny on top of a cup of ice, you’d be able to check the cup for an indicator. If the penny is on the bottom, the ice must have melted and thus your food must have thawed. If the penny is still on top, all was well.
Why the penny in the freezer trick doesn’t work
Besides being annoying to set up (you need to freeze the cup of water for hours and then come back and lay a coin on top), there’s a much bigger problem: A coin in a cup is a terrible indicator of what has happened, temperature-wise.
First: Ice floats. I don’t know why everyone assumes the penny will slowly sink, giving you a thermometer-like metric of how melty your freezer got, because ice does not work that way. I froze a cup of water and placed a coin on top of it, for science, and I’m going to show you what actually happens when the ice melts.
What actually happens when you put a coin on a cup of ice
First, the freezing portion: How long does it take to freeze a cup of ice solid? I used an IKEA KALAS cup, which holds about eight ounces of liquid. Surprisingly, I found it took less than two hours for the water to freeze. The entire cup was not frozen at that time, but the top was solid enough to place a coin and know that it wasn’t going anywhere. I left the cup in the freezer for several days before I got around to performing the melting portion of the experiment, so it was fully frozen solid before we started.
11 a.m.: I remove the cup from the freezer. I also place several ice cubes into a second cup, which I will use for a comparison—you’ll see why in the next section. I keep the cups on my desk, so I can watch them throughout the day as I write.
12 p.m.: I am out to lunch.
1:02 p.m.: The ice has melted all around the edge of the cup. There is now a puck of ice, with the penny still on top, floating(?) in the middle of this pool of water. So much for the theory that if the penny is on top, the ice didn’t melt.
1:21 p.m. I hear a clink. The penny just slid off the ice floe (I swear I didn’t bump it) and is now at the bottom of the cup, sideways, in the crevasse between the ice and the side of the cup. A moment later, it has slipped underneath the ice and is now lying flat at the bottom. So much for the theory that if the penny is at the bottom, the ice must have fully melted.
There you have it: For the first two hours, the penny stayed at the top. And then, in the blink of an eye, the penny was at the bottom. (Don’t read too much into my timeline; in an actual power outage, the freezer would be colder than my office, and the ice would melt more slowly. The size and shape of the cup would likely also affect when and whether the penny slides to the bottom.)
How to read the results of the penny-in-the-freezer trick
What does this tell us? If you leave a penny on top of a cup of ice, and return to find the penny at the bottom, you know some amount of melting occurred, but you don’t necessarily know how much. But if you return to find the penny still on top of the cup of ice, melting may or may not have occurred.
So this trick is useless at telling you how much melting occurred, and is unreliable at telling you whether melting occurred. The only thing you know for sure is that if the penny makes it to the bottom, there was some amount of melting. But there’s a better way to get that information.
There’s a much easier way to find out if your freezer lost power
Photo: Beth Skwarecki
Just throw a few ice cubes into a cup before you leave. If they melt, you’ll find a block of solid ice when you return. If they partially melt, you’ll find a few ice cubes stuck together by the melted-and-refrozen water at the bottom of the cup. It takes about ten seconds to set this up on your way out the door, as opposed to freezing a cup of water for hours and then trying to remember to put that coin on top.
The University of Nebraska suggests keeping a zip-top baggie of ice cubes in your freezer. (A closed cup or container would also be fine.) They recommend the baggie or closed container because ice can evaporate slowly over time. In an open cup, your ice cubes might get smaller or even disappear without having melted.
You can also, of course, look at the items in your freezer and see if they still look and feel the way they usually do. Does your ice cream still have that fluffy ice cream texture, with scoop marks at the top where you’ve previously served yourself? Or is it a rock-hard block of refrozen ice cream soup?
What to do if you know (or suspect) that your freezer lost power
If you’ve been reading up on storm preparation and food safety tips, you may have noticed that the major safety organizations don’t include the penny trick (or even the ice cube trick) in their official guidance. That’s because it was never their idea in the first place.
A full freezer will keep its contents cold for up to 48 hours if the door stays closed (and for 24 hours if it’s half full), according to the FDA’s power outage food safety tips. A brief outage of just a few hours isn’t going to ruin your food.
But if you did lose power for a day or more, there’s no reliable way to know whether your food is still good to eat. Foodsafety.gov emphasizes that you should not taste or eat the food to see if it’s still good; the rule is when in doubt, throw it out.
If the power is still out, you can refer to this chart from foodsafety.gov. Items that are only partially thawed, with ice crystals still present, can often be refrozen. Items that were fully thawed should almost always be discarded. The exceptions are things like muffins or hard cheeses that are safe to eat at room temperature normally.
