With Hurricane Irma beginning to touch down in Florida, many residents are opting to temporarily evacuate rather than endure the storm. Unfortunately, property damage and displacement aren’t the only potential consequences of Irma and other disasters. If a home loses power while occupants are away, it might turn on again before they come back. That means food in the refrigerator and freezer can reach room temperature before cooling back down, inviting microorganisms that cause foodborne illness.

According to the Atlanta Journal-Constitution, there’s an easy way to tell if foods have been thawed and then re-frozen: Freezing a cup of water and placing a penny on top acts as a cheap sentry for power outages. If the water remains frozen, the penny will stay on top of the ice. If it melts, then the penny will begin to sink into the cup.

The newspaper points out that some Facebook posts recommend doing this with a quarter, but a penny is preferable: The copper conducts heat better. If it’s moved, then you know food has been exposed to warmer temperatures that could cause bacteria to thrive.

This hack works whether you’re in the path of a storm, need to vacate due to fumigation, or plan to be away for a weekend.

[h/t AJC.com]