Product review (5 stars): This is ingenius. A simple but very clever way to know if your freezer is keeping things frozen properly. Smart!
You may order this, expecting some complicated sort of "meter" (at under $10, lol) and then be surprised to see what this is and how it works.

It is not just "simple"--it is actually very ingenious. And you might think "so simple there must be some way I could have just done this on my own". But really, this device is completely worth the small cost of it. I admire the design and cleverness!

Basically, it's a little translucent plastic container with a cap and a little Velcro tie that you can strap to something in your freezer (or just stand the tube upright if that works).
1. You fill the tube with water to the line shown, then put it in the freezer.
2. The water freezes.
3. You open it and put the red ball in on top of the frozen water. Put the cap back on.
4. That's it!  If you open the freezer and the ball is still there--everything is working fine.  If the red ball is middle way down, then something in your freezer was defrosting but it probably hasn't been long enough to cause problems with food safety.

If you look in and the ball is at the bottom of the tube--even if the water has frozen above it--you can see that your refrigerator defrosted. If the power was out you have the explanation for it--and also know that it takes about 2 hours for the ball to reach the bottom of the tube. That may be too long for things like meats to not be kept frozen.

That's how it works! Simple, reusable, and very, very clever!