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Explain to me that it is opposite day. On opposite day, everything you say is reversed. "No" means "yes", "is" means "isn't", and so on. Because you are trying to explain to me it is opposite day on opposite day, the task isn't simple. For example:

  • If you tell me it is opposite day, you will be saying that it isn't opposite day, and therefore won't convince me.

  • If you tell me it is not opposite day, I will simply believe and agree that it is not opposite day.

What can you tell me to convince me, without a doubt, that it is opposite day?

Edit: For those asking, the sentence's meaning is reversed. So "I am going to tell a lie" turns into "I am not going to tell a lie".

Edit #2: For those claiming the riddle is too broad, opposite day has been revised to changing meaning. There is a specific task and specific parameters.

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  • $\begingroup$ Perhaps too easy, but would you be convinced by obvious false statements? i.e. "It's not raining" (when it is), "That wall is black" (it's white)? $\endgroup$
    – berry120
    Commented Feb 5, 2018 at 14:02
  • $\begingroup$ Does every part of what you say get reversed or just the meaning of every phrase? ie If I say something like "I'm not going up" does every part get reversed so it becomes "I am going down" or does it just reverse the sentence as a whole to "I am going up"? $\endgroup$
    – Chris
    Commented Feb 5, 2018 at 23:21
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    $\begingroup$ @Chris Reverses meaning, or else everything you say wouldn't be affected. $\endgroup$
    – Assafi Cohen-Arazi
    Commented Feb 5, 2018 at 23:22
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    $\begingroup$ There are many answers here which, even if not what you intended, seem like they would fit the bill. You should have some objective criteria, even if arbitrary, for determining the "best" or most "right" answer, so that we're not just assembling a collection of alternate answers. If one of the responses is suitably "right", please don't forget to $\color{green}{\checkmark \small\text{Accept}}$ it. If not, some responses to the answerers to help steer them in the right direction would be helpful. $\endgroup$
    – Rubio ♦
    Commented Feb 5, 2018 at 23:37
  • $\begingroup$ @Rubio I was going to wait a couple of days to find what I thought was the most "suitable" answer. My personal answer was not as clever as some that have appeared here, and therefore I want to give others a chance before I accept so soon. $\endgroup$
    – Assafi Cohen-Arazi
    Commented Feb 5, 2018 at 23:43

13 Answers 13

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I would say

I am going to tell a lie. Today is not opposite day.

Which means

I am not going to tell a lie. Today is opposite day.

Both will convey the same meaning that today is opposite day.

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  • $\begingroup$ I guess this is same as the second example in the question. If it is said in a normal day, it won't be true, because 'today is not opposite day' is not a 'lie' $\endgroup$
    – Ahmed Ashour
    Commented Feb 5, 2018 at 10:20
  • $\begingroup$ "What can you tell me to convince me, without a doubt, that it is opposite day?" That means the question is on convincing someone it's opposite day in an opposite day. It doesn't concern about other days. $\endgroup$
    – AeJey
    Commented Feb 5, 2018 at 11:28
  • $\begingroup$ It's similar to "You should believe that today is opposite day." $\endgroup$
    – Phil M Jones
    Commented Feb 5, 2018 at 14:48
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    $\begingroup$ But "I am going to tell a lie" ends up being the truth, which is not allowed. $\endgroup$
    – Don Hatch
    Commented Feb 5, 2018 at 19:16
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    $\begingroup$ Wouldn't 'lie' become 'truth'? $\endgroup$
    – vero
    Commented Feb 5, 2018 at 22:43
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I would say

If you ask me if today is the opposite day, i would say yes

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    $\begingroup$ It took me a few seconds to work this one out, it's actually a very clever answer. +1. $\endgroup$
    – F1Krazy
    Commented Feb 5, 2018 at 14:59
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    $\begingroup$ So if I don't ask you that question you won't respond? :p $\endgroup$
    – StevenWhite
    Commented Feb 5, 2018 at 20:05
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I would say:

Today is not opposite day.

Wait a second, it's already stated in the puzzle that ...

Well, let me interrupt you right there. I think the second example is a trick. The puzzle clearly states that saying something will be reversed on "opposite day", so saying "Today is not opposite day" on opposite day will become "Today is opposite day" which solves the problem.

Now, let me elaborate on that trick I mentionned. I think what OP meant by "If you tell me it is not opposite day, I will simply believe and agree that it is not opposite day" is that if you tell him that after the inverting has occured, meaning you originally said "it is opposite day". Also, notice that the second example doesn't look like the first example.

After all, it's a puzzle.

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    $\begingroup$ My brain hurts... $\endgroup$
    – Assafi Cohen-Arazi
    Commented Feb 5, 2018 at 3:12
  • $\begingroup$ So does mine... $\endgroup$
    – ibrahim mahrir
    Commented Feb 5, 2018 at 3:42
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I would say:

One plus one is not equal to two. It may not or may be possible for you to work out whether today is opposite day from that statement.

which is the reverse of:

One plus one is equal to two. It may or may not be possible for you to work out whether today is opposite day from that statement.

and since:

One plus one is equal to two, you can tell that the statement is reversed.

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  • $\begingroup$ @JonathanAllan I interpret 'everything you say is reversed' to be 'every statement is switched with its logical negation', which would give the sentence I used. $\endgroup$
    – boboquack
    Commented Feb 5, 2018 at 0:38
  • $\begingroup$ Pretty sure you are right... $\endgroup$
    – Jonathan Allan
    Commented Feb 5, 2018 at 0:50
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    $\begingroup$ Shouldn't possible to for you be possible for you to ? Or is my English broken ? $\endgroup$
    – Rafalon
    Commented Feb 5, 2018 at 12:01
  • $\begingroup$ @Rafalon yes it should be $\endgroup$
    – boboquack
    Commented Feb 5, 2018 at 20:09
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Easy, since I am talking on opposite day

Every other day is opposite day

This works

  • first because at face value it is untrue so therefore today must be opposite day.
  • second because if you exchange 'every other' for its opposite 'no other' it becomes true, therefore again today must be opposite day.
  • third, if you reverse the meaning of the sentence, then today is the opposite day.
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    $\begingroup$ +1 This was also my idea. $\endgroup$
    – F1Krazy
    Commented Feb 5, 2018 at 14:27
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If the definition is that all statements are to have their opposite meaning then simply state:

Opposite day isn't the name given to a day when one's statements have opposite meaning.

Maybe for clarity (to make it not just a lie on non-opposite days) you could add:

...and today isn't such a day!

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No need even calculations, I would say: On opposite day

"A black cat is not a black cat today"
"5 is not 5"
"True is false"

and normal day

"A black cat is a black cat today"
"5 is 5"
"True is true"

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Upon reading them, none of the other answers has so far convinced me that today is opposite day...

I claim that your question is, not only vague, but fundamentally vague. That is, you cannot word it in any way that precludes a funny interpretation instead of the one you meant.

I guess part of the problem is that it is never clear what exactly you mean by opposite. For instance, what is the opposite of "this apple is green":

  • "this apple is not green"
  • "this apple is red"
  • "this apple is ripe"

Or worse, what is the opposite of a tautology?

Another problem is that in one of your examples the listener believes what is said, while not believing the other statement. Thus, you left the door open for me to interpret any "solution" in such a way as to remain unconvinced. In fact, no solution will convince me, because it is NOT opposite day today.

A final thought: If you add some rules to the question, then there is an actual puzzle. There has to be an unbreakable rule about how the listener perceives the veracity of the speaker...

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  • $\begingroup$ I appreciate your feedback, but this is not an answer, and therefore shouldn't be in the answer section of the post. $\endgroup$
    – Assafi Cohen-Arazi
    Commented Feb 5, 2018 at 23:57
  • $\begingroup$ As it stands, this is really more commentary on the question than an answer to it. Having said that, "Nothing I might say will for sure convince you." might well be a defensible answer to the puzzle. I'd recommend you reframe this response in that way; otherwise it will likely be deleted as "Not an answer". $\endgroup$
    – Rubio ♦
    Commented Feb 6, 2018 at 0:21
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Hmmm...your first statement seems to be contradictory-

'If you tell me it is opposite day,you will be saying that it isn't opposite day and therefore won't convince me'.

The key phrase here is 'you will be saying that it isn't opposite day.' In order to make that assertion and accept that statement,you need to recognize that what you are being told is the opposite-in other words,it is opposite day. Your statement's truth is conditional on recognizing that it is opposite day.

So we end up with a paradox

Either

a)You're told it is opposite day.Since you state you know that this means it isn't opposite day,you have to have recognized that it is opposite day.

b)You're told it is opposite day.Since you don't recognize it as opposite day,then you can't state you know that it means it isn't opposite day.

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    $\begingroup$ The point was the the sentence "It is opposite day" is false on both opposite day and any other day, so it yields no information in the case that the listener does not know which type of day it is today. $\endgroup$
    – Jonathan Allan
    Commented Feb 5, 2018 at 0:08
  • $\begingroup$ There is no paradox because when you say it is opposite day, it means it isn't on a regular day. $\endgroup$
    – Assafi Cohen-Arazi
    Commented Feb 5, 2018 at 1:46
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I would say

Black equals white. It is not opposite day.

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It's a regular day!

it is a regular day. On a regular day, everything I say is straight-forward.
"Yes" means "no", "isn't" means "is", and that's it. Since I'm telling you it is a regular day on a regular day, the task is simple. Because:

1. When I tell you it isn't a regular day, but I won't say it is a regular day, and therefore convince you.

2. When I tell you it is opposite day, you will argue and refuse to believe it's opposite day.

However I highly doubt that whatever I ask you can convince you that it isn't a regular day.

oef, after trying to reverse everything you asked to fit it as an answer I bet I made a million mistakes. However, I think this is the answer you were looking for!

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When we used to play this at school, some people said

It is opposite day... I mean, it is not opposite day!

Technically, this is not correct, but

You would just think I messed up inverting all my statements, so you will think it is opposite day.

Not a perfect logically correct answer, but in practice it would propably work.

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Maybe the following

Hand them the following note:

Hello,

Tomorrow is opposite day.

Signed, SyntheticAbyss
Postmark date: yesterday

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  • $\begingroup$ Leaves open the possibility that yesterday was opposite day, and you were telling them that today wouldn't be. $\endgroup$
    – Ben Voigt
    Commented Feb 6, 2018 at 2:45

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