The Grey Labyrinth is a collection of puzzles, riddles, mind games, paradoxes and other intellectually challenging diversions. Related topics: puzzle games, logic puzzles, lateral thinking puzzles, philosophy, mind benders, brain teasers, word problems, conundrums, 3d puzzles, spatial reasoning, intelligence tests, mathematical diversions, paradoxes, physics problems, reasoning, math, science.


The 9 Minute Egg

After years of diligent honing of your puzzle-solving skills, you are finally rewarded with a position at the presitious Princeton Institute for Advanced Studies. Unfortunately, it is as a cook, but you have to start somewhere.

One December morning after a particularly heavy snow storm, the power fails. Fortunately, there is still an old wood stove with which you can prepare most of the professors' breakfasts. However one eccentric Mathematics professor with a great deal of power and influence at the institute has a peculiar breakfast item which now poses a problem. He likes a single egg boiled for exactly nine minutes.

You aren't wearing your watch, and all the clocks in the building are electric. You are able to find two exquisite hourglasses, able to pricely measure in hand-crafted swiss sand seven and four minutes, respectively.

How quickly using only these two hourglasses can you provide the professor with his egg?

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