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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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.
Sally's older brother has evidently been travelling in two distinct inertial reference frames.
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)
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.
Two different answers:
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")
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...