For this reason, you should always put a coin in your freezer

The Science behind The Coin-in-the-Freezer Hack-Behind The Stuff.

This tip is basically about understanding how ice behaves. Ice melts above the freezing point, and when it refreezes, it can alter its shape, density, or structure. The coin is a silent witness to any thawing that might have occurred so you will know whether your frozen items are safe to eat.

A coin placed on a small cup of ice gives you an indication. If you lose and regain your power supply, the melted ice forms into water under which the coin sinks. If your freezer cools again, the water would freeze into ice, and the coin is now trapped deeper into the ice. Now, its sinking inside the water indicates that it is your freezer whose contents thawed.