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Two people were born on the same day, same year, and share the same birthday.

One is 2 years old, the other is only a few days old.

What's going on here?

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11 Answers 11

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They were born on

the New Year's Eve (not necessarily of the western variety), and one of the families is using the East Asian way to measure age.

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    $\begingroup$ @Saeïdryl because one of them were 1 year old already at the moment he/she was born, so next year this person goes on 2. This answer can be more accurate if this New Year's Eve comes from the lunar calender. $\endgroup$
    – Shane Hsu
    Commented Mar 21, 2018 at 10:19
  • $\begingroup$ @ShaneHsu I confused myself by reading New Year's Eve instead of New Year's Day on Wikipedia... I get it, Bass' "New Year's Eve" refers to Asian Eve and not December the 31st. But that's not clear! $\endgroup$
    – Saeïdryl
    Commented Mar 21, 2018 at 10:29
  • $\begingroup$ @Saeïdryl that's a fair point. Edited. $\endgroup$
    – Bass
    Commented Mar 21, 2018 at 11:11
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    $\begingroup$ But aren't we assuming that the children belong to different families? The answer below using science seems more appropriate. =P $\endgroup$
    – An old man in the sea.
    Commented Mar 21, 2018 at 18:00
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    $\begingroup$ @Anoldmaninthesea. Did the question ever specify "twins"? The way I'm reading the revisions, it always said "two people", which can be pretty easily interpreted to mean "two people [from different families]". $\endgroup$
    – anon
    Commented Mar 23, 2018 at 20:43
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We'll just apply a bit of science assuming they were born in same year as well:

The younger kid was put in a spaceship that rotated around the galaxy at ultra-high speed and came back to earth.

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    $\begingroup$ this can be the only correct answer $\endgroup$
    – minseong
    Commented Mar 21, 2018 at 10:03
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    $\begingroup$ Since days are no doubt calculated in Earth's reference frame, and we can assume that even a spherical, frictionless baby would malfunction if subjected to constant acceleration exceeding 20g, it would take almost three weeks to accelerate an initially stationary baby to the required speed. Therefore this answer is slightly inaccurate, and the other baby must have been born on the already relativistic spaceship. $\endgroup$
    – Bass
    Commented Mar 21, 2018 at 16:18
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    $\begingroup$ "...a spherical frictionless baby..." only on puzzling.SE would one expect such a phrase $\endgroup$
    – Darren H
    Commented Mar 21, 2018 at 16:50
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    $\begingroup$ I joined this site solely so I could upvote the phrase "a spherical, frictionless baby" $\endgroup$
    – Daniel Beck
    Commented Mar 22, 2018 at 23:39
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    $\begingroup$ @DanielBeck Welcome to puzzling.SE, where we compete with WorldBuilding.SE to create spherical, frictionless babies. $\endgroup$
    – workoverflow
    Commented Mar 25, 2018 at 9:38
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Apply some other bit of science, assuming they were born at the same time,

.. but on Mercury.
According to Mercury's Wikipedia entry, Mercury rotates extremely slowly, so An observer on Mercury would therefore see only one day every two years.

The two people are the same age, they are both two years old, which on Mercury is equal to very few (one, to be precise) days.

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Is it lateral-thinking?

They are born on the same day but not on the same year.

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  • $\begingroup$ Different years means that it is not the same day, otherwise the puzzle should have read: "same day of the year" and not just "same day". $\endgroup$
    – Bjarke Freund-Hansen
    Commented Mar 22, 2018 at 10:32
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    $\begingroup$ @BjarkeFreund-Hansen It is subjective, i.e. if I say "same hour" I never implied it was on the same day. So instead of posting about time relativity and twin paradox I decided to play on the lack of informations and lateral thinking. $\endgroup$
    – Saeïdryl
    Commented Mar 22, 2018 at 10:40
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    $\begingroup$ @BjarkeFreund-Hansen: It's open to interpretation, which is often where the trick of a puzzle is located. Arguably, "the same day" could mean "Friday", "the 7th", "Jan 7th", or "Jan 7th 2017". You just can't be sure. $\endgroup$
    – Flater
    Commented Mar 23, 2018 at 10:03
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location location location

one lives at the north/south pole, where each day is 6 months long

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Sudden loss:

One child died a few days after being born. The family refuses to acknowledge the age of the lost child as if it was alive and so while one is celebrating it's 2nd year of life the other will forever remain only a few days old.

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This could be an historical question

The change from the Julian calendar to the Gregorian calendar didn't happen at the same time in every country. If one person was born on February 29th in a country that was still under the Julian calendar, and the other was born on the same day in a country that was now on the Gregorian calendar, then one would have a birthday every year, while the other had one every four years, despite being born on the same day.

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It's a

Twin Paradox!!
Applying Einstein's Special theory of Relativity!

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    $\begingroup$ Welcome to Puzzling.SE! You may find the tour interesting, where you'll earn an easy badge. I edited your answer with >! to hide your answer from others until they want to see it. $\endgroup$
    – Chowzen
    Commented Mar 21, 2018 at 12:59
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Could it have anything to do with

being born on February 29th? Where I come from people would say that if you were born on 29th February you can only celebrate your birthday that day which means 1 year only adds up every 4 years?

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  • $\begingroup$ Welcome to Puzzling.SE! Could you edit your answer to include spoiler tags, so as not to spoil your solution for anyone who wants to have a go at the puzzle themselves? Thanks! $\endgroup$
    – F1Krazy
    Commented Mar 21, 2018 at 15:43
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    $\begingroup$ Around 732 days later, this might explain saying one baby is "not yet one year old", but not really that the baby is "only a few days old"... $\endgroup$
    – aschepler
    Commented Mar 22, 2018 at 12:07
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It's all about space and time!

They were born at same time but not in the same planet. Suppose that both were born at their respective planets at the same universal time (which on Earth would be, for example, March, 22th 1988).

But at the moment of answering this question both planets had completed different translations around their star. So if person A is at Earch he/she is now 30 years old, and if person B is at a planet which translation time is a little slower (so the planet have completed only 28 laps around it's star, rather than 30) he/she will have only the age of 28.

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    $\begingroup$ (Even for lateral-thinking puzzles—which this is NOT—the solver is not given free license to invent their own rules or scenarios. Especially for puzzles not tagged "lateral thinking", the right answer to a puzzle will be the one that uses what the puzzle gave you or hinted at, without inventing facts, rules, or interpretations out of thin air to make a "solution" work. Puzzle posters can't close every loophole… and shouldn't have to.) $\endgroup$
    – Rubio ♦
    Commented Mar 23, 2018 at 7:47
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One was born

In the year 1 BC

And the other

Year 1 AD

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