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Sally smiles at her older brother and says, "Happy 30th birthday!"

He shakes his head and replies, "I'm not 30 yet, but you are!"

How is this possible?

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They are

twins, and the brother was born earlier than Sally on the same day.

They are located on different time zones, either talking via video call or simply standing on different sides of a border between two time zones.

It is already their birthday on Sally's time zone, but not yet on the brother's time zone.

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  • $\begingroup$ (+1): I had the same thing in mind! $\endgroup$ – Mr Pie Apr 20 at 9:17
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Sally's older brother has evidently been travelling in two distinct inertial reference frames.

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    $\begingroup$ Best answer. The other responses rely on debatable pedantry but this one is pure physics. $\endgroup$ – afeique Apr 20 at 15:18
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    $\begingroup$ @afeique Definitely not - the reference frames do not need to be inertial; moreover, for the siblings to have anything like meaningful realtime conversation, the frames cannot be inertial the whole time. $\endgroup$ – Radovan Garabík Apr 20 at 19:37
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    $\begingroup$ It seems to work. Traveling in two distinct inertial frames implies that there was a change of reference frame that was not inertial. $\endgroup$ – Jay Apr 21 at 4:14
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    $\begingroup$ Or maybe he's been orbiting a supermassive black hole. $\endgroup$ – n00dles Apr 21 at 21:50
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Maybe

he was born on Feb 29th, Leap Day and so has had about 1/4 the number of birthday's you would expect he's had.

Or if we want to be more legal...

They are twins born on Feb 29th (he was born first) and this is a non-leap year and Feb 28th. He is somewhere that considers March 1 as his official birthday (UK for example) and she is somewhere that considers Feb 28th as her official birthday (New Zealand or Taiwan, for example)

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  • $\begingroup$ I did consider that, but from a legal standpoint would "I'm not 30 yet" be false? $\endgroup$ – rybo111 Apr 20 at 10:29
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    $\begingroup$ Not sure, but it is the premise of Gilbert and Sullivan's "The Pirates of Penzance"! $\endgroup$ – SteveV Apr 20 at 14:56
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    $\begingroup$ @SteveV IIRC, Gilbert and Sullivan very carefully worded it as "his 21st Birthday", and not "21 years old" $\endgroup$ – Chronocidal Apr 20 at 16:01
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Try mine

Sally and her older brother are actually twins, their mother were in a cruise while labouring those two, the older brother came out first while in the 1st of March side of the Pacific Ocean, while she was born in the 28th February side of the ocean. I found the answer here.

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Two different answers:

1)

They are Korean, and he got a European or USA citizenship and is making a point by reminding her the age reckoning is different (though strictly speaking, she would not be speaking of 30th birthday, but of "being 30 year old")

2)

Very easy. Why wouldn't this be possible? People often say things that are not true. E.g. making simple mistakes when intoxicated after a birthday celebration, or just unsuccessfully trying to be funny or just pointing out he is feeling younger...

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the answer is: Sally has two brothers

a younger brother who is 26 and an older brother who is 28. Sally says: “Hi, have you met my two kid brothers? Let me introduce my younger brother Adam and my older brother Brian”. She smiles at her older brother.

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  • $\begingroup$ she smiles at her older brother.... $\endgroup$ – Radovan Garabík Apr 22 at 8:56

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