A medical examiner works in Cadaversville, a place where the dead may sometimes wake in the off hours then die once more. While awake, they are not zombies and don't eat people. They just seem to be sleepwalking and are known to initiate conversations. Luckily, the refrigeration unit in the morgue freezes the body to below 32°F if one wakes up while the morgue is closed. But every time a cadaver wakes up, the medical examiner has to cure them later so they do not reanimate. There is no way of telling if a cadaver had awoken once it dies again, and the refrigeration unit does not indicate whether or not it had to freeze.

How can the medical examiner make a simple mechanism, machine, or apparatus which would be placed within the cooler and indicate whether or not water in the mechanism (and the cadavers) has frozen?

closed as off-topic by Alconja, Peregrine Rook, AzaNov 26 '16 at 17:47

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• Use an off-the-shelf min-max thermometer? :) – jpa Nov 25 '16 at 11:08
• Oh, come on now, the right solution is obviously nuke from orbit. Otherwise we will be facing the zombie apocalypse! – Loren Pechtel Nov 25 '16 at 11:25
• I actually do not have any well established answer for this puzzle. I will use the answers and experiment to see what method works best. – Tommy Woldt Nov 25 '16 at 15:54
• Couldn't you just put an iPhone in each cooler and if there's a new selfie posted to Instagram, the cadaver woke up? – Ian MacDonald Nov 25 '16 at 15:59
• I'm sorry, what? Is this a genuine problem you're having and you're crowd sourcing a solution? – user1566694 Nov 25 '16 at 16:07

For the purposes of the description, I'm going to be using measurements, but there's no real need for exact numbers in this — it's just to add clarity.

1. Find a 100ml glass.
2. Put 90ml water in the glass.
3. Top the remaining 10ml with oil.
4. Place the glass in the cooler.

When the water freezes, it will displace the oil and cause it to overflow. If the cooler is opened and the water is frozen or there is fewer than 100ml in the glass, the freezer had turned on.

Make sure the diameter of the glass is sufficiently small such that the surface tension of the meniscus does not hold the oil in.

Alternatively:

1. Find a 100ml glass.
2. Put 50ml water in the glass.
3. Dust flour on top of the water.
4. Place the glass in the cooler.

When the water freezes, it will push the flour up the glass. As it thaws, the flour will retreat to the edges and stick at a "high-water mark". If this mark is higher than the current observed level of the water (or the water is still frozen), the freezer turned on.

• I thought the norm here was to wrap answers in spoiler tags? – LSpice Nov 25 '16 at 20:01
• I find that spoiler tags on a puzzle without a known solution are a little unnecessary. – Ian MacDonald Nov 25 '16 at 21:32

My suggestion would be to:

Fill a completely rigid but brittle plastic bottle full to the brim with water (the bigger the better).

This would work because:

As water freezes, it expands, so if the water has frozen, it would burst the bottle. You can try this experiment at home! Since the bottle is rigid it won't bend for the ice, but since it is brittle it will crack.

• Works for me.. Good thinking. – Timme Nov 25 '16 at 10:36
• like my forgotten beer in the freezer – lois6b Nov 25 '16 at 10:40
• You're assuming it's enough to burst, which might not be the case. You can sidestep that issue altogether - leave the cap off, and then check if anything overflowed out. – Rubio Nov 25 '16 at 11:03
• @Rubio i don't think this is reliable, i could imagine the ice going up straight, and sinking back down when it liquifies, so it maaaay stay in the bottle? i might be off on this one. also: just realised: take the bottle, close it, but near the top poke a hole in the side. the water going out there can not slide back down in any way, so yeah this might be even more reliable than the exploding version. – Timme Nov 25 '16 at 12:03
• @Rubio I'll edit my answer, but if the plastic is perfectly rigid but also brittle, it will snap under the pressure. – boboquack Nov 25 '16 at 20:59

Note: this is just a funny answer

DO NOT TRY THIS AT HOME

Have you ever put popcorn in the freezer? Well don't. Put unpopped popcorn in the freezer and it cracks open (sometimes enough to pop like under heat). Simply put, stick a bag of popcorn in the freezer. It will have the effect of showing you if the corpse froze (after all, flash-freezing will do far more popping potentials than just sticking in the freezer) and it will also make the cadaver vault smell a bit better. Plus, when the guy wakes up for a bit he'll be greeted by the smell of popcorn. It's less cruel than the smell of their own decaying flesh. Note one again, this is a humorous answer. While I have seen this work irl (I accidentally put popcorn in the freezer overnight), it isn't expected to actually serve as a cadaver-detection system. Way too many flaws, including evidence contamination.

• @TommyWoldt you're welcome. It would actually work; however, I doubt you want to use that method in your morgue. Murdered corpses might get contaminated. I'd hate for this method to ruin an important case. – The Great Duck Nov 26 '16 at 5:55
• It might actually work, I'd have to get it approved by the Cadaver Ethics Committee and consult the lead detective. We do have the methods and materials to sterilize a kernel and place it within a container within the cooler. – Tommy Woldt Nov 26 '16 at 5:58
• @TommyWoldt Great! Glad to help. :) – The Great Duck Nov 26 '16 at 6:00

He could

Place a pot of fleas under a microscope.

This would work as

Fleas would die at 32 degrees Fahrenheit. So if he wants to check if it went to that temperature he can look at the microscope to see if all the fleas are dead.

You could also

Place a small animal in there that would die at that temperature, like a type of goldfish or something.

True, this could be seen as animal cruelty, but when the world is in danger of a zombie apocalypse, I don't think anyone will complain.

• animal brutality :( – lois6b Nov 25 '16 at 12:08
• @lois6b, what sort of behaviour do you expect from a brutal animal like Beastly Gerbil? :-) – LSpice Nov 25 '16 at 20:01

Fill 80% of a ziplock bag with water, and place into a container. Take a stick about the diameter of the bowl and carve a very shallow bowl into one end. Balance the stick on the edge of the container with a marble in the bowl in the stick outside the container. Move the stick back and forth so it's balanced on the edge of the bowl with one end on the water bag and the other end outside the container.

If the water freezes it'll expand the water in the bag, which will tip the stick up and the marble will fall.

This is a little touchy, but it's reset-able. You could instead secure a glass rod across the bag and it would break when frozen, or do something substantially similar.

There are also freeze indicators you can buy, or you can use air pressure mechanisms since air pressure depends on temperature. Bur these seem more involved and expensive.

He could

Place a penny or small heavy object on the surface of the ice (within a container such a a glass)

This works

if the ice has thawed and melted again, the coin would have sunk some distance, and now be not on top

• the question is about the reverse situation: it freezes only if necessary, and you have to detect if it has frozen sometime along the night – Federico Nov 25 '16 at 14:39

He could

Place a special appratus that captures pressurized air!

How does it work?

Take a pipe and connect one end with a pressure gauge and then fill the pipe with mercury(90%). Then to the other end connect a one-way air valve which only lets air inside. Make sure this is done in normal temprature. Now keep it where you want to check the temprature change. As the freezer freezes the area, the mercury will contract demanding air from one way valve...now as the temprature reaches normal, mercury will expand again but the air will not escape so indicating a reading in pressure gauge.

This method is non-destructing so, you can use this gear/equipment any number of times given that it only tells that freezing has occured but not how many times.

• This method is great, but it would also indicate upon the normal temperature fluctuations within the cooler. Though experimentation can be performed to calobrate each PSI measurement to a temperature – Tommy Woldt Nov 26 '16 at 18:10
• @TommyWoldt I think he means normal temperature of the refrigerator. – The Great Duck Nov 27 '16 at 22:26
• @thegreatduck I mean the temprature in which you will take reading...can be anything above 32F,but should be quite significant. – Mukul Kumar Nov 28 '16 at 2:43