Product review (4 stars): Wish the interpretation markings were printed on it but it is a useful little device for determining if your freezer thawed.
Yes, you can make one of these yourself with a penny and a cup.  But this is easier,  has a strap to hang and does not need to be balanced on top of your other food.

I wish the interpretation markings were printed on the device rather than the cardboard header,  taking a star for this oversight.  It is pretty straightforward,  if the freezer loses power, the ice melts and the ball will move down in the tube.  The farther down it moves, the more melted the stuff in your freezer became before it refroze.