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A while back, I was at a sporting event, holding my ticket and waiting for the race to begin, when a woman came up beside me and commented, "You know, the runner in lane one and lane two both have the same exact birthdate."

"Really?" I asked. "Same year, even?"

"Of course," she laughed, "it would be hardly notable if it weren't."

"And they aren't twin brothers or anything?"

"No." Her hair bounced as she shook her head. "Not even barely related. But what's really something is that the runner in lane three has the same birthday too. Day, month, and year."

"Wow!" I responded, "That is really impressive. Three of them, here, and they're all exactly the same age!"

"I didn't say that," the woman commented, with a smirk. "In fact, the second racer is three months older than the one in lane one."

"What? That doesn't make any sense at all. You said they had the same birthdate."

"They do."

"Down to the year."

"Yup."

"But he's--" I gestured at the second lane of the track, "older than him."

"Yes!" The woman was grinning now. "And the third runner is three months older than he is! That's six months older than the first runner."

I crossed my arms. "Okay, clearly you're trying to mess with me here. I know how brainteasers work. What's the trick? Are we dealing with different calendars here? Or, like, the international date line? Or--"

The woman cut me off with a frown. "No, no calendar change, no Chinese New Year, no geographic shenanigans, no leap years, and!" she raised a finger, "no time-dilation absurdity here either. It's just the facts."

Fed up, I exclaimed, "Oh yeah!? I suppose the fourth one has the same birthday but is nine months older than the first, huh?!"

Snapping, "Don't be ridiculous," the woman turned on her heel and stalked away, leaving me to stare at the racetrack and the competitors and ponder. How could what the woman said be true?

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6 Answers 6

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The racers are all:

Thoroughbred horses

These always have:

An official birthdate of 1 January in the Northern Hemisphere, or 1 August in the Southern. This is to enable bloodlines to be tracked for breeding purposes.

Therefore all the racers in this horse race have the same official birthdate, even though they may have been born 3 or 6 months apart...

This has other quirks in consequence too:

Notably, a horse born the day before the official birthdate will turn '1 year old' the next day! Also, since the age of a horse determines which races it can compete in at some events (e.g. 3 years old, for the Triple Crown), this means stable owners often prefer horses born shortly after the official birthdate, as they are usually more mature and stronger (therefore better racers) than their end-of-year counterparts.

This is a similar analogue to the oft-quoted remark that elite sportsmen and sportswomen tend to have birthdays towards the start of their school years, since being slightly older than their end-of-year-birthday peers they are likely also more physically developed, more quickly; this gives them a slight competitive edge all the while teams are decided based on age, helping them to develop their skills and their game. There are - of course - notable exceptions; for example, Lionel Messi (born 24 June - 2 months before the 31 August end-of-school-year - and yet Ballon d'Or winner on multiple occasions). Notably though, Messi often says in interviews that at school and in his first tryouts for professional clubs his smaller stature (in comparison to his peers) did disadvantage him in the eyes of the coaches...

The woman's scoffing at the suggestion that one could be 9 months older than the first:

Could be due to the first horse being just below the racing age cutoff - if the other horse really was 9 months older it would be ineligible for the race and shouldn't be competing at all!

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    $\begingroup$ I swear I've seen a puzzle with the exact the same resolution on this site (+1) $\endgroup$
    – Anon
    Commented May 14, 2020 at 5:13
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    $\begingroup$ Of course, rot13(ure fgngrzrag gung "vg jbhyq or uneqyl abgnoyr vs vg jrera'g" vf xvaq bs n aba-frdhvghe, fvapr vs guvf vf pbzzba cenpgvpr jvgu ubefrf, gura vg'f uneqyl abgnoyr nalubj. Vg jbhyq va snpg or cerggl pbzzba naq rira rkcrpgrq gung zbfg bs gur ubefrf va n tvira enpr jbhyq or nebhaq gur fnzr ntr.) $\endgroup$
    – Darrel Hoffman
    Commented May 14, 2020 at 14:29
  • $\begingroup$ Unfortunately, that cutoff doesn't just affect your chances of becoming a pro-athlete. It affects your chances of being diagnosed with ADD/ADHD or other behavioral issues as a child $\endgroup$
    – Kevin
    Commented May 14, 2020 at 15:06
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    $\begingroup$ So, hard shenanigans it is. These are not the actual birthdates but just how they're registered. $\endgroup$
    – paddotk
    Commented May 15, 2020 at 10:54
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The woman is referring to someone as "older" or "younger" based on his age since conception ("gestational age"). His birthdate of course is his date of birth. Nine months would be ridiculous, as she noted — even if the "oldest" runner were born after a very long pregnancy, it would mean the "youngest" would be born after too short a pregnancy to make his survival likely (assuming the runners are human, which wasn't stated).

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    $\begingroup$ Not a bad guess, but this doesn't have anything to do with three-month gestation periods. $\endgroup$
    – Exal
    Commented May 14, 2020 at 4:58
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Some of the racers are so fast, they've traveled at near light speed, and thus have aged differently (due to relativity).

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    $\begingroup$ The question eliminates this possibility: "no time-dilation absurdity here either". $\endgroup$
    – Rand al'Thor
    Commented May 14, 2020 at 20:15
  • $\begingroup$ So does that make Usain Bolt older or younger than all of us? $\endgroup$
    – Ian MacDonald
    Commented May 14, 2020 at 20:18
  • $\begingroup$ @Randal'Thor it's not "time dilation absurdity". It's very natural and normal time dilation. $\endgroup$
    – nosson
    Commented May 14, 2020 at 20:23
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    $\begingroup$ Very normal yes. Happened to me yesterday. $\endgroup$
    – sehe
    Commented May 15, 2020 at 7:45
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She never says

They have the same birth day as each other.

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    $\begingroup$ "What? That doesn't make any sense at all. You said they had the same birthdate." "They do." $\endgroup$
    – paddotk
    Commented May 15, 2020 at 10:56
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Solution:

According to Concise Oxford English Dictionary the definition of date is:
the day of the month or year as specified by a number If going by this definition, the dates spoken about where particular days in a year, disregarding the month.

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  • $\begingroup$ Welcome to PSE! The question clearly states they have the same birthdate "down to the year". That means day, month and year $\endgroup$
    – melfnt
    Commented May 15, 2020 at 13:16
  • $\begingroup$ Thank you. I did not overlook the part with "down to the year". I attempt to clarify: for example, the first racer may be born on January 1st, 1990. The second April 1st, 1990. The third July 1st, 1990. Going by the definition of date that either the month OR the year is combined with the ordinary number of the day, the explanation makes perfect sense. $\endgroup$
    – simon
    Commented May 15, 2020 at 13:24
  • $\begingroup$ "Down to the year" means day, month and year. They cannot be born in different month $\endgroup$
    – melfnt
    Commented May 15, 2020 at 13:47
  • $\begingroup$ That's a question of definition and semantics. Obviously the riddle cannot be solved if the definitions of the terms date etc. would be interpreted as normally understood. So the solution must involve a creative or commonly unknown re-definition of at least one of the terms used. $\endgroup$
    – simon
    Commented May 15, 2020 at 13:54
  • $\begingroup$ @simon Nice try, but I think you're misinterpreting the definition. The day of the month will necessarily include that month. For example 5th of March, or 3rd of February. The day of the year is just one day in that year. For example 114th day of 2020 was on April 23rd, etc. So you can't make different dates fit in that way. This is also why the problem specified that they were all born the same year, exactly because of the ambiguity of March 7th being the same date in many years. $\endgroup$
    – Amorydai
    Commented May 15, 2020 at 15:42
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One is born from the mother at full term. There other two exited the mothers' womb prematurely at 6 and 3 months respectively?

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