Inspired by @DEEM's Grandpa Mystery puzzles (e.g. here and here). Be sure to check them out!

Now we all know that Grandpa is a genius... but sometimes he says the most absurd things!

Today he told me:

Did you know that $6$ is prime?

I replied:

$6$ is not a prime number, as it is equal to $3\times 2$. Oh wait, are you referring to the gesture your friend showed you nearly last week?

He then made one final reply before he went off to have a brownie:

No. I never said number.

What is Grandpa trying to say?


20 minutes later...

I walked up to Grandpa. Why was it taking so long for him to finish his brownie? I soon found out he fell asleep at the table.

Grandpa, wake up! I think I might know how $6$ is prime. If I am right, then you are very clever!

He suddenly jolted upwards and said:

What? You might think it's a letter? No no no no no.

And he went back to snoring. Poor Grandpa. I think it is best I leave him alone, now. $$$$ Hint 2.

I began to steal one of his brownies as he was asleep, but that was when I heard him muttering to himself. He said:


I had no idea what he meant by that, but when I looked at the brownies, they looked identical... like twins! So I just left them alone... $$$$ Hint 3. Last Hint before I will declare a 50 rep bounty.

When Grandpa woke up, I asked him:

Grandpa, what do you mean by prime?

He replied:

Well, how long did I rest for exactly?

I checked my watch, and said:

Just under a full hour.

He looked baffled and made one last reply before eating the last brownie I was going to save for myself:

Well, your watch could be wrong. I slept for just over 5 minutes. That's what I mean by prime.

But I wasn't wearing it when Grandpa first asked me this ridiculous question... $$$$ Super last hint.

Well, I know the secret now! I figured it out!

No, I'm not going to tell you the answer here. I'm just going to say my watch isn't wrong... I mean, it could have been, but now I know the answer. You see, you have to pay attention to the words. Grandpa said, "No. I never said a number," and it doesn't make sense because he just did, right? The number $6$. That was why I found it so confusing...

...but he did say, "No."

Told ya my Grandpa was a genius, and now the question is: What am I trying to say?

  • $\begingroup$ Hah... the title even rhymes :) $\endgroup$ – Mr Pie Sep 2 '18 at 3:23
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    $\begingroup$ Thanks for continuing the Grandpa Mysteries. My Grandpa thanks your Grandpa! $\endgroup$ – DrD Sep 2 '18 at 10:33
  • $\begingroup$ @DEEM I've always loved those Grandpa Mysteries, as much as my Grandpa loves brownies :D $\endgroup$ – Mr Pie Sep 2 '18 at 10:35
  • $\begingroup$ @downvoter may you please explain why you downvoted? Did I do something wrong or did you not like the riddle (because this might not be your taste)? I don't care about reputation — I just want to do what is right by the community :) $\endgroup$ – Mr Pie Sep 3 '18 at 7:31
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    $\begingroup$ Since you asked, I downvoted this just now, as I usually do, when things appear "seriously broken as written". By that I mean I am taking the point of view of someone coming in later (like myself), who just looks at the question as it currently stands, and the accepted answer as it currently stands. $\endgroup$ – deep thought Nov 28 '18 at 2:19

My sniper longshot

6 doesn't represent the number 6. That's why Grandpa says "No. I never said number (6)".


We know it's not a letter and not a number, so we are left with symbols.


Unicode U+212X is the third row of Unicode characters, if X is 6 we get Omega. Omega, in maths, is used to denote how many prime divisors a number has. So Grandpa is not saying 6, but Unicode 6 is prime.

Previous answers (none is the actual one)

Six has 3 letters. 3 is prime.


6 = G => G = 7 and 7 is prime.


TV Primetime. But it is between 8-11 PM.


6 is between Twin Primes (5-7).

  • $\begingroup$ No, that's not it. The gesture statement refers to how $6=3$ (here), but Grandpa said no. Bad Grandpa :D $\endgroup$ – Mr Pie Sep 2 '18 at 3:25
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    $\begingroup$ Umm... Maybe rot13(T rdhnyf frira naq frira vf cevzr). Are the previous post necessary? I haven't read them. $\endgroup$ – Nicolás Magno Sep 2 '18 at 3:29
  • $\begingroup$ The previous post is only necessary for that part. Also, the rot13 is not correct, but it can pass! I might have to consequently add more to the puzzle for the intended answer... aannnnnd done! $\endgroup$ – Mr Pie Sep 2 '18 at 3:31
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    $\begingroup$ Let's give others some time to solve the real answer (including me :) ). $\endgroup$ – Nicolás Magno Sep 2 '18 at 13:29
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    $\begingroup$ That means I am 6. Therefore, I am prime! Lol $\endgroup$ – Nicolás Magno Sep 2 '18 at 14:04

As a wild guess,

Per Wikipedia, "Prime, or the First Hour, is a fixed time of prayer of the traditional Divine Office (Canonical Hours), said at the first hour of daylight (approximately 6:00 a.m.)". If Grandpa is Christian, he could mean that 6 am is time to say prime, as opposed to lauds or terce.

Although this matches only one of the tags...

  • $\begingroup$ I only used the knowledge tag for knowledge of prime numbers. Also, the letter X can stand for christ (or being christian), but no letters (on their own, that is) must be involved. It was a good try, though. I've added a second hint :) $\endgroup$ – Mr Pie Sep 2 '18 at 4:31
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    $\begingroup$ @user477343 Anything related to rot13(gjva cevzrf svir naq frira)? But then the other tag doesn't work out... $\endgroup$ – Mariia Mykhailova Sep 2 '18 at 6:00
  • $\begingroup$ It is related to that rot13, but not the particular pair of numbers you mentioned. Remember, that information is from the hints, not the actual riddle itself. One of the tags don't really work only because the hints have made it a little easier to directly solve. $\endgroup$ – Mr Pie Sep 2 '18 at 8:35

Wrap-up: The Making Of $\,$$6$ is prime? Another Grandpa Mystery (not made by @DEEM, this time)

This is not a solution to the puzzle, but provides notes from its poser. This type of answer has been approved by the community.

Caution: This post may contain spoilers.


Well, firstly, thanks heaps to @DEEM for his/her Grandpa Mysteries. Those puzzles were in fact my inspiration. I don't really have a favourite tag, but I do watch out for rhyming riddles; though I have started to watch out for lateral-thinking because of these Grandpa puzzles! In fact, one of my personal favourites is this one, which is why I included the line, "Now we all know that Grandpa is a genius..." $$$$

Steps of Creation

I first began thinking about a number, and trying to find an interesting, whacky property about that number. Fortunately enough, it was the very first number I thought of: the number $6$. Now the accepted answer has been accepted, but that was not actually the intended answer. I accepted it because I really liked the answer, and it could have actually been the answer, too. I will not reveal the answer, here, but I hope the steps of the creation provide a big hint.

Because this was going to be the lateral-thinking type, I decided to try and find something useful in SIX and not $6$. And that's when I came up with the answer and made a Grandpa puzzle out of it. I then just started playing with words, realising that words like "No" can stand for "number" without you actually saying "number".

I worded it this way to make people think that $6$ is prime with definitions excluding prime number... but not necessarily. It's only a number if you refer to $6$ as a number (as opposed to a word). Even though the actual prime that $6$ is equal to is in fact a number, it is reached from knowing that $6$ is not referred to as a number, because numbers behave much more differently than words (even if they might mean the same thing; i.e., Eight = $8$ but Eight $\neq 2^3=8$).

After that, the following hints were just a bit of wordplay. Grandpa says "no" a lot in the Hint 1 from what I just mentioned. (And the last sentences in Hint 1 also have a cool property to reveal the actual prime number itself.) Time is also used for correspondence, and "twins" is also a clue. (Note that $20$ in "$20$ minutes later" sounds like "twin".) And then it gets a bit weird how the super last hint mainly refers to the actual puzzle and not the previous hints.

In the end, it all seemed to work well. $$$$


I just used DEEM's puzzles to kind of imitate the overall style of the Grandpa Mystery. $$$$


I am happy that I was able to make a decent Grandpa Mystery puzzle. Also, thanks to this post for the construction of this answer! :D

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    $\begingroup$ And my Grandpa loved this puzzle. In fact he said that he is coming up with another one to acknowledge your puzzle. $\endgroup$ – DrD Sep 3 '18 at 11:22
  • $\begingroup$ @DEEM looking forward to it. In the meantime, Imma get some more brownies... $\endgroup$ – Mr Pie Sep 3 '18 at 13:18

A simple answer is that:

Your Grandad is senile.

  • $\begingroup$ But Grandpa is a genius! $\endgroup$ – Mr Pie Sep 2 '18 at 10:12
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    $\begingroup$ there is a fine line between senility and genius... $\endgroup$ – JMP Sep 2 '18 at 10:15
  • $\begingroup$ Well, either way, Grandpa has nothing wrong with him, except that he falls asleep too quickly and eats too many brownies. Besides, this explanation does not match Hint 2. $\endgroup$ – Mr Pie Sep 2 '18 at 10:24


Grandpa is in his sixties, so he is saying that $6$ is prime [the state or time of greatest vigour or success in a person's life].

  • $\begingroup$ Close... it doesn't match Hint 2 and it's not about age, but there is an element of your answer that is close to arriving at the correct one :P $\endgroup$ – Mr Pie Sep 2 '18 at 10:14

Something very silly

He asked "Do you know?". So the simple answer is "Yes" or "No". As 6 is not prime, "no"

  • $\begingroup$ Rot13(Fb gur sbphf vf abg gur "Cevzr" cneg naq vs lrf, gura jung vf gur sbphf?) $\endgroup$ – Quark-epoch Sep 2 '18 at 15:56
  • $\begingroup$ No, that is not the answer. How a phrase sounds like is of no concern (i.e. for the intended answer); it is more literal than that. To say the least, Grandpa is not asking a question. It was me who was asking the question because I couldn't figure out the meaning behind Granpa's silly statements. But I know, now :P $\endgroup$ – Mr Pie Sep 2 '18 at 23:48

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