The professor for my software engineering course in college posed a problem with a non-tech answer similarly.
You have a cabin in the woods, with electricity and a freezer. You go hunting and bag a deer with much more meat than can be used in one visit, so you freeze it for use on a future visit. But you want to know while you are away if the electricity fails for any length of time so that the meat may have thawed and spoiled.
After the class spent most of a period arguing over remote monitoring systems and temperature sensors and the like, the professor revealed the simple answer. Freeze a block of ice in the freezer and mark its height. If it is shorter when you return, the power was out so some of it melted.
His answer is a clever one that is very cheap and effective, but personally I'll be the first to admit that I would've been among that group talking about going the more complicated route because I figured:
1) A block of ice will occupy room that could've been used for more deer meat(I suppose the size wouldn't matter, of course)
2) A temperature sensor would let you know right away if the power failed, instead of finding out after the fact and forcing you to clean out a freezer full of rotting meat
3) The sensor may also act as a silent alarm system. If the temperature changes suddenly then goes back to normal, it's possible that an intruder opened it up looking for money / food / valuables.
I realize that misses the point of the discussion since the professor was going for the easiest possible solution, but we're hackers here :)
Plug an electric analog clock on the same circuit as the freezer. If it is wrong by more than a minute or two (normal inaccuracy) then you know the power was off and for roughly how long. If the power was off for an exact multiple 12 hours you'd have a problem, but a) that's unlikely and b) you could get a slightly fancier clock with a date as well as a time component.
Ice evaporates, how do you tell the difference between evaporation and power outage? Sure there should be more loss in the case of an outage but you don't know the exact evaporation rate to expect.
Your computer has a setting on whether or not it should turn back on after power loss, build a low cost circuit that does the same(turns off in case of power loss) and you are done.
Place something like a coin on top of the ice. If the ice melts, the coin will submerge and be inside the ice if it re-freezes. (Assuming you put the ice in a container).
.. and the nice thing is that it's a close representative of what you're trying to measure. If it's warm enough to melt a significant portion of ice, it's warm enough to spoil your food.
I was thinking of some sort of canary that would trip if power turned off, but that would only tell you if the power ever went off, not how long it was off. No sense wasting a fridge full of meat if the power was off for a few seconds.
A clock that requires electricity but displays the time mechanically with a date on it would also suffice, as you could measure the delta between what it displays and what the time really is (accounting for normal inaccuracies).
A weakness of the clock idea is that the power could be off multiple short periods; too short to allow the meat to spoil, but enough in sum to make the clock pretty far off.
Battery-backed microcontroller logging all power off events and temperature changes :)
If the battery has run out, then there has been enough long power outages that the meat is probably spoiled. If the battery hasn't run out, you have a nice log of what happened when.
Not that I think its better than the ice solution, but theres certainly techie options that you could build for $10 or so.
Cover the deer with crushed ice. If it is still covered when you get back, the meat is still good. Even if the power was out for a while and some of the ice melted. (A large freezer filled with meat and ice will stay cold for quite a while. Even if some of the ice melts and refreezes, the temperature stayed freezing.)
This and many other posts are all good answers, but the point of the problem was simply to be able to think non-technologically and non-computer, not always an easy task for a class of computer science college students naturally disposed to think that code and clean code is the solution to all the world's problems. The precise best way to set up a telltale with ice is not the main concern. :)
Where do you get this block of ice? (Admittedly pretty easy to acquire/make if it's smallish.) And where would the students get their electric gadgets?
Assuming you took a truck hunting, get rid of the problem altogether. Stick the meat in the freezer and stick the freezer in your truck and take it home with you where you can not only monitor it personally, but still enjoy the meat whenever you want. The only concerns then are keeping the freezer interior cold enough for the trip back (pretty easy) and the space at your place.
I did something similar for a deep freezer I had in my garage. I drank half a small bottle of water then froze it sideways. After it was frozen, I put it upright and in a place it can't fall over. If the water goes flat to the bottom, then I know the freezer was powered off.
"During the space race back in the 1960's, NASA was faced with a major problem. The astronaut needed a pen that would write in the vacuum of space. NASA went to work. At a cost of $1.5 million they developed the "Astronaut Pen". Some of you may remember. It enjoyed minor success on the commercial market.
The Russians were faced with the same dilemma.
They used a pencil."
The idea is to show outcome-oriented thinking. The desired outcome wasn't to use a pen, but rather, to simply write or record.
That anecdote happens to be false though. The Astronaut Pen was made independently of NASA and was sold to them (and Russia) like it's sold to the public. Wood pencils pose trouble in zero-g (graphite dust, broken tips, flammable wood, etc).
I knew that the anecdote was false and forgot to add that. The point wasn't to show up NASA or research. The point was to show outcome-oriented thought.
It was still very poor outcome-oriented thought. There are dangers associated with pencils that are solved by a pen. Space flight's outcome should be safety, so regardless the answer won't end up being a pencil.
I think it still works fine as a basic metaphor to help illustrate the idea. The metaphor doesn't need to be 100% inline to do that and expecting it to seems a bit pedantic.
Speaking of pedantic, a flight's outcome is not really safety, nor was space flight what the folklore mentions. It was addressing the requirement to write/record in space.
But the Italians say Si non è vero, è ben trovato.
I've read a book from a former astronaut though, that tend to prove there's some truth to the legend; it was about the vacuum cleaner used in Mir. The NASA devised a sophisticated way to clean up the Space Shuttle; the Russians simply used an ordinary vacuum cleaner from the nearest supermarket.
I've dug out the book :) p. 99 it says (rough translation):
"I remember an anecdote from 1982. That year, the vacuum cleaner used in the saliout 7 station failed. I had the opportunity to admire the simplicity and efficacy of the Russian method. An ordinary vacuum cleaner was bought from Star City supermarket, sent to Baïkonur via the next plane and loaded into the next Progress cargo ship. Once aboard the station, it was plugged in, and worked perfectly. Problem solved. The whole operation costed 5 rubles (1 US $)".
Other posters have already mentioned the urban legend that this comprises.. but on a slight rant, I hate it when this anecdote gets rolled out because this is the sort of short-sighted thinking that gets interesting research canned.
One person felt long ago(can't find the exact quote) that there was once a world market for maybe five computers, and others thought that lasers and x-rays would be nothing more than a sham.
For a considerable time the laser was considered a solution in search of a problem. Now, of course, they have a huge number of vital uses and nearly everyone in the developed world owns several in one form or another.
For that matter it took a long time for computers to have an impact on productivity. For a while people wondered whether personal computers actually made business sense. Now, in the era of mass global computerized communications the advantages are hard to ignore.
If the pencil tip broke while writing in space, it could easily mess up astronauts nostrils, instrument panels etc.
The pen approach takes care of corner cases.
You have a cabin in the woods, with electricity and a freezer. You go hunting and bag a deer with much more meat than can be used in one visit, so you freeze it for use on a future visit. But you want to know while you are away if the electricity fails for any length of time so that the meat may have thawed and spoiled.
After the class spent most of a period arguing over remote monitoring systems and temperature sensors and the like, the professor revealed the simple answer. Freeze a block of ice in the freezer and mark its height. If it is shorter when you return, the power was out so some of it melted.