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Two sisters were born on the same day in the same year, yet they aren't twins. None of their other sisters share the birthday. What's going on? Explain.

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closed as too broad by Rand al'Thor, Gareth McCaughan, Dan Russell, Deusovi Sep 23 '16 at 3:38

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ possible duplicate of Miss Danner and the Elmer Boys $\endgroup$ – Bozman Nov 6 '14 at 15:04
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    $\begingroup$ Not a duplicate of either $\endgroup$ – Joe Nov 6 '14 at 15:16
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    $\begingroup$ @kaine I think he explicitly tried to avoid the answer in the first one with "None of their other sisters share the birthday". $\endgroup$ – TheRubberDuck Nov 6 '14 at 16:17
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    $\begingroup$ @EnvisionAndDevelop I would agree that that is likely even if I only saw the question. This is despite the fact that they could easily be brother-sister-sister triplets or they could be born near midnight so one sister is special. I meant, however, that as OP posted his own answer that is different, it isn't a duplicate and shouldn't be closed. $\endgroup$ – kaine Nov 6 '14 at 16:22
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    $\begingroup$ However, could it possibly be considered "too broad"? $\endgroup$ – Aza Nov 6 '14 at 18:04

12 Answers 12

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When you say "two sisters", one thinks it means "they are each other's sister". While it actually only means "each one has a sister/brother". So the easiest answer is "they are not in the same family"

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    $\begingroup$ This is exactly as true as Colonel Panic's answer. I actually like it better. $\endgroup$ – TheRubberDuck Nov 6 '14 at 17:05
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The two sisters aren't twins, they are either triplets (or greater) with the other triplet being a brother (or bothers if greater than triplets).

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  • $\begingroup$ That's one G.Weinberg riddle ! $\endgroup$ – Fabinout Nov 6 '14 at 17:05
  • $\begingroup$ Definitely the most reasonable answer here, and probably the intended solution. $\endgroup$ – 6005 Nov 9 '14 at 10:34
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The sisters are nuns. Their shared birthday is a coincidence.

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    $\begingroup$ Am I the only one bothered by somebody asking a question and then answering his or her own question 4 minutes later? $\endgroup$ – Warlord 099 Nov 6 '14 at 18:58
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    $\begingroup$ Answering your own question is acceptable on SE. Not so on the puzzling site? I like that he did because the OP can always be outvoted, like on the other sites, by a better answer because answers are all in competition. $\endgroup$ – geoO Nov 6 '14 at 20:07
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    $\begingroup$ @geoO but in Puzzling, there's usually not much that makes an answer distinct. Now nobody can give this answer without basically stealing, because it's only one or two sentences anyway! Instead of taking the same premise and explaining it better, other answers have to come up with new premises. $\endgroup$ – TheRubberDuck Nov 6 '14 at 20:59
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    $\begingroup$ @geoO A key part of that was "4 minutes later". Answering it is one thing, but at least let others have a chance to answer it. Otherwise it seems like asking the question just to gain cheap rep. $\endgroup$ – Warlord 099 Nov 6 '14 at 21:53
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    $\begingroup$ @Warlord099 pardon me. I wanted to leave the puzzle to everyone, but it looked liked the post was going to be closed as a duplicate (of a different question with a different answer), so I hastily answered it myself. $\endgroup$ – Colonel Panic Nov 7 '14 at 10:41
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Same father, different mother.

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  • $\begingroup$ @ColonelPanic answer is better though. $\endgroup$ – Taemyr Nov 6 '14 at 15:16
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    $\begingroup$ These would be half-sisters, which technically is different to just "sisters" $\endgroup$ – Joe Nov 6 '14 at 15:17
  • $\begingroup$ @Joe Which is why ColonelPanic's answer is better. $\endgroup$ – Taemyr Nov 6 '14 at 15:17
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    $\begingroup$ this is what I thought of first, and this is the simplest most logical answer $\endgroup$ – Ayyash Nov 7 '14 at 14:45
  • $\begingroup$ The other way around is also possible. Turns out there even is a name for it: heteropaternal superfecundation $\endgroup$ – kasperd Nov 8 '14 at 17:43
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they are step sisters. One is adopted into the family.

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Two identical twin brothers marry two identical twin sisters. They each have baby girls born on the dame year-month-date. While we'd consider the girls cousins, they are genetically sisters.

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14
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Same day, same year but what about a different month? e.g. 1st January 2000 and 1st December 2000 (more than 9 months apart)

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    $\begingroup$ I think "same day" very strongly implies same calendar day, which includes the month. If your answer were true it'd be a cheap trick. $\endgroup$ – geoO Nov 6 '14 at 20:09
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    $\begingroup$ Alternatively, Same day of the week. Both were born on a Monday. One on a Monday in December, one on a Monday in January. $\endgroup$ – Lyndon White Nov 7 '14 at 6:10
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    $\begingroup$ A variant on this might involve time travel or time zones. $\endgroup$ – Michael Nov 7 '14 at 23:33
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They have the same genetic parents but different surrogate mothers.

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One or more has died, so you could say they WERE twins but not ARE twins.

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  • $\begingroup$ +1 because this was my second thought, after the "not having the same parents" angle. $\endgroup$ – COTO Nov 6 '14 at 23:58
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Having sister companies founded in the same day at the same is not unusual.

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It's very similar to Colonel Panic's answer:

The two ladies are nursing sisters; their shared birthday is a coincidence.

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In the same vein as Taemyr's answer, but reversed:

TL;DR: One Mother, possibly Two Different Fathers
Superfecundation: Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse. The term superfecundation is derived from fecund, meaning the ability to produce offspring.
Heteropaternal superfecundation occurs when two different males father fraternal twins.

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    $\begingroup$ those are still twins; doesn't count $\endgroup$ – ratchet freak Nov 6 '14 at 17:56
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    $\begingroup$ @ratchetfreak is correct -- they are fraternal twins: "Fraternal (dizygotic) twins develop from two eggs fertilised by two sperm, and are no more alike than individual brothers or sisters (siblings) born at different times". Also, heteropaternal sisters would only be half-sisters. $\endgroup$ – Doktor J Nov 6 '14 at 18:05
  • $\begingroup$ There is another, rarer circumstance where two ova from different cycles are fertilized. I don't know if those are considered twins or not, though. $\endgroup$ – KSmarts Feb 20 '15 at 16:48
  • $\begingroup$ @ratchetfreak I think you are right, but... If two girls (from separate pregnancies) that have only one parent in common, are not sisters but only half-sisters, I wonder if two heteropaternal girls from the same dizygotic double pregnancy (conceived naturally with two men and one woman) are only half-twins, and not twins. $\endgroup$ – Jeppe Stig Nielsen Aug 15 '17 at 13:24

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