If your freezer stopped working for some time while you were on holiday, you'll want to know, since your food might have harmful bacteria. Find out by keeping half a bottle of frozen juice upside down.
As the King of Random mentions in the above video, you will have to first freeze the juice in the bottle, then invert it. This way, the frozen liquid will be tightly packed at the top, defying gravity. Once you come back, if it's still the same way, then you know the freezer has been working well the whole time. But if the juice has dropped to the bottom and then frozen, that's a sign that the freezer had switched off long enough for the temperature to drop to thawing point and then start up again.
The King of Random's method of using two bottles seems like overkill though. A better option would be to freeze a bottle upside down first (propping it up with something) while you're still at home, so the liquid freezes near the mouth. Then invert it so it's the right side up, keep it in any corner of your freezer, and forget about it until you return.
5 Clever Life Hacks You Need To Know [The King of Random]