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Warning: To answer this riddle you need to have a thorough understanding of how time works.

Two twins were born during the same year, on the same month, and the same day, exactly 10 minutes apart. By the time twin A was 100, twin B was younger by more than 10 minutes, fairly slightly. Neither twin died during the 100 years.

How could Twin B be younger than Twin A by more than 10 minutes?

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  • $\begingroup$ Define your terms. In particular, what do you consider younger to mean? (And if you give an answer in terms of age, what do you consider age to mean?) $\endgroup$
    – Peter Taylor
    Commented Oct 13, 2014 at 14:25

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Due to time dilation, moving faster will make you age slightly slower. This is most commonly mentioned with astronauts and space calculations, but it can also be applied to lower speeds.

Technically, if Twin A normally drives his car at 50mph over the course of his lifetime because he wants to save fuel, whereas Twin B dashes around at 70mph because he's always running late, Twin A will have experienced more time than Twin B, so Twin B would be slightly more than 10 minutes younger after a century.

Thanks to Clint's comment below, there is a similar effect that could happen - gravitational time dilation. If Twin B lives in Washington DC (250ft above sea level) but Twin A moved to Denver (5000ft above sea level) at a young age, time will pass slightly more slowly in Washington because it's closer to the gravitational centre of the earth, again resulting in Twin B having experienced less time.

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  • $\begingroup$ I was going to say this, but I couldn't type so fast. Also, a similar effect occurs if they live at different altitudes. $\endgroup$
    – Clint Eastwood
    Commented Oct 13, 2014 at 14:05
  • $\begingroup$ @ClintEastwood updated to mention gravitational time dilation as well as speed time dilation :-) $\endgroup$
    – Joe
    Commented Oct 13, 2014 at 14:14
  • $\begingroup$ The gravitational part was what I was looking for! In particular I expected "Twin B was an astronaut" $\endgroup$
    – warspyking
    Commented Oct 13, 2014 at 15:20
  • $\begingroup$ @warspyking I see that it's been edited out of your question, but "most realistic" would technically mean that just living in Denver instead of Washington is "more realistic" (more believable) than being an astronaut :P Not that it matters $\endgroup$
    – Joe
    Commented Oct 13, 2014 at 15:42
  • $\begingroup$ (I just noticed you meant the gap only widened a little bit, not that the older twin became younger.) $\endgroup$
    – user88
    Commented May 2, 2015 at 14:59
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Time dilation is possible, and was my first thought, but I have an easier answer. Twin A and Twin B were born 42 years apart. They also have no relation to the two twins that were born ten minutes apart.

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  • $\begingroup$ Very clever but no, good try! $\endgroup$
    – warspyking
    Commented Oct 14, 2014 at 18:07
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Twin A has since moved to the other side of the world, with the timezone differences they technically differ in age by 12h10m or 11h50m, depends on where they started :)

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  • $\begingroup$ "twin B was younger by more than 10 minutes, fairly slightly" - I'd say 12 hours is much more than 10 minutes, not "fairly slightly" $\endgroup$
    – Joe
    Commented Oct 13, 2014 at 14:06
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    $\begingroup$ Over the course of 100 years, 12 hours could be thought of as "fairly slightly" $\endgroup$
    – red-X
    Commented Oct 13, 2014 at 14:07
  • $\begingroup$ That's fair enough, I just think that the way the question is worded, the "fairly slightly" almost certainly refers to 10 minutes, not to 100 years, and trying to interpret it otherwise is forcing it a little too much heh $\endgroup$
    – Joe
    Commented Oct 13, 2014 at 14:09
  • $\begingroup$ The fact that they live in different time zones doesn't account for any difference in age as they've lived the same amount of time. $\endgroup$
    – Bob Jarvis - Слава Україні
    Commented Nov 10, 2014 at 12:11
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It just happens. Depends on how much and how fast you travel, I guess. If you travel more often, you will be younger. The faster you travel, the better.

Other possibility is that the earth just significantly slowed spinning, so a day is longer, which means that a minute is longer. But this only works with the old definition of minute as a fraction of the mean solar day, not if you take the newer one, based on cesium 133.

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