When you leave your home you will put vegetables, fruits and food. But if the electricity cut the bacteria will grow in the freezer. Especially salmonella will grow. Then those foods will not suitable for consume.
For this you need:
? a cup
Fill water to the ball and keep it in the freezer to freeze. Then put the coin on the ice and keep it in the freezer again.
After you arrival the home check the coin before consume the food which was in the fridge.
If the coin was in the top or middle of the ice it will be fine as the electricity was not cut or it was cut for little time. Therefore water has melted little amount.
But if the coin is on the down then you have to not use those foods. Because electricity has cut for long time and there will be a bacteria growth.