Scenario: I want to hard-boil a single whole egg with the least amount of effort and active cooking time. I don't own an egg steamer and don't want to buy another kitchen gadget that takes up space without being used most of the time. I also don't want to cook the egg in a pot of boiling water because it seems wasteful to heat up so much water for a single egg. (Edit: I only have 2 sizes of stove hobs, so the smallest pot I own is the size of the smallest hob: 14,5 cm / 5.7 inch diameter) However, I do have a thermos flask with 1 liter capacity and an opening big enough to put an egg inside. Yesterday I put an egg in and filled it with about half a liter of freshly boiling water. After 25 minutes only a few millimeters of the outer eggwhite were cooked and the inside was completely liquid. I took the egg directly from the fridge, so that might have contributed to the fail. I also thought about putting the egg directly into my electric kettle, but if the shell cracks it would be a nightmare to clean the kettle afterwards. I know that microwaving an egg has an insanely high risk of the thing exploding in your face. There are many reports of people receiving really bad burns to their faces and hands. Is it even possible to cook an egg in a thermos flask? Is there maybe another method I overlooked? I'm willing to wait about an hour for the egg to finish cooking, as long as I don't have to actively monitor the process.  Update: I tried again with the 2 methods that appealed to me the most: Joe M's method: egg in thermos flask. I only have the "keep things warmer for a little longer" kind of flask, not the "keep things hot for hours" vacuum kind, but if I fill it up to the brim with boiling water and gently tilt it every 5 minutes to even out the temperature insite, I can get a hard-boiled egg in 20 minutes. Tjaden Hess' method: steaming in a pot. This works like magic. I made a little "throne" for the egg from aluminium foil and boiled a cup of water in the electric kettle before pouring it into the pot, which was heating up in the meantime. After pouring the water and adding the lid, I could turn the hob down to the lowest setting and forget about the egg for 10 minutes. It came out perfect and this method required the least effort and resources by far. 
Yes, it is possible to cook an egg in a thermos. After thinking about the physics some, I decided to try this out.  In particular, I considered that the egg is maybe 50ml in volume; with 500ml of boiling water, it shouldn't be a problem to transfer plenty of heat to raise the 50ml egg from 4°C to 82°C on the outside (less in the middle, yolk is cooked by around 70°C). The important thing is to make sure the egg is immersed in water that stays around 80-85°C for the duration of the cooking cycle - about 10-15 minutes.  So:  A thermos must be a true Thermos® - or at least, a very close kin, double walled vacuum style.  The "keep your hot food hot for 8 hours" type, not just a coffee tin that will keep your coffee sort of warmish while you hop on the bus to work. You lose a few degrees of heat when you pour the water into a cold thermos - so I tested pre-warmed vs. cold thermos. How much water is needed? I had a 500ml Thermos® and two 300ml Thermos®, so that seemed an apt comparison.  My experimental setup: 1 pre-warmed 500ml thermos, 1 pre-warmed 300ml thermos, 1 room temp 300ml thermos.  One kettle of boiling water with about 1.5L water.  Three "American large" eggs, pulled from the refrigerator just as I started the setup but well after I put the kettle on (so only out for a few minutes, no meaningful raise in temperature from the 4°C fridge). Method: I poured some extra boiling water from a separate kettle about 2 minutes before the main kettle came to a boil into the 500ml and one 300ml thermos.  Then right when the main kettle came to a boil, I poured those two out.  Then I put eggs in all three (the warm 500ml one was very hard to do that in, it was very warm!).  Then I poured 100°C water (temped from my electric kettle) into the three, filling each to the fill line (the metal band) and as quickly as I could topped them each with the screw on top. Then, I waited 15 minutes. I left them alone - no mixing, no touching, me sitting at my computer ignoring them until my timer went off.  After that time was up, I opened all three and immediately temped the water in them.  Then, I quickly removed the eggs, placed them in bowls, and cracked them with a spoon, then cut with a knife. My results:  The 500ml setup cooked an egg precisely how I like it - maybe even a tiny bit overdone if anything.  Whites fully cooked, yolk medium to medium well with a great yellow/orange color.  The water in the thermos after 15 minutes was 80°C, which is right where it should be.  I'd pull this out a bit earlier next time, maybe after 10 minutes.   The 300ml "prewarmed" cooked a pretty decent egg also.  It was less well done on the inside than the 500ml, but many people would like an egg boiled this way, and it's well within my "good egg" range.  Whites firmly done, yolk not dry at all and a great yellow.  The water in the thermos was 73°C, so a bit below the "done whites" temperature by this point, but likely it was within acceptable cooking range (80°C-85°C) for the 10 minutes needed.   The 300ml "room temp" was inconclusive.  The egg was decent, but the color much lighter yellow - but also a much smaller yolk entirely, oddly small and pale compared to what I'm used to.  It wasn't clear if it was underdone or okay; it wasn't runny, certainly.  I don't know if the color was due to cooking or not.  The water was 72°C, so basically the same as the other one - maybe the prewarming made a trivial difference, or maybe it made no difference at all; I don't necessarily think it's likely to, but who knows.    I also separately tried an egg in 1L of water not in a double walled thermos.  This totally failed - the egg was still basically liquified; clearly partially cooked, but not something I'd want to eat, runnier than scotched eggs by a good bit.  The water had dropped to about 60°C by 15 minutes later, so it clearly was not in the optimal cooking temperature. In conclusion: yes, it's possible to cook an egg in 500ml of water in a thermos, provided you have a proper thermos that's capable of keeping the water at a high temperature!  It likely is possible to boil an egg harder than I prefer, but the more water you use, the better for that. My thermoses (the 500ml, and one 300ml, egg for scale):  