Isn't one way of envisaging this:

Make a table showing the answer you get.

if the___________If you picked
liar is________Jon______the other
Jon___________?__________?
the other______?__________?

To satisfy both questions you must one cell must be different from the other three. But a single question can only distinguish between the rows OR the columns.

Thus 'Are you Jon?' gives

if the___________If you picked
liar is________Jon______the other
Jon___________No__________No
the other_____Yes_________Yes

'Does Jon lie?' gives

if the___________If you picked
liar is________Jon______the other
Jon___________No__________Yes
the other______No__________Yes

and so on.

If you ask both questions in succession then you get four combinations.
No,no - you picked Jon and Jon is the liar
No,yes - you did not pick Jon, and Jon is the liar
Yes,no - you picked Jon, but Jon is not the liar
Yes,yes - you did not pick Jon and Jon is not the liar

Or, as a table

if the_____________If you picked
liar is__________Jon________the other
Jon___________No,No_______No,Yes
the other______Yes,No_______No,No

If you are interested in who is Jon, that's the question.
However, if you are concerned with whether Jon is a liar, you should look for another question.

>Yeah only took me a whole week

You did eat, did not you?

That will only tell you if the person you are addressing is Jon or not, but not the identity of the liar.

Any question with a yes/no answer won't work, because there are only two possible answers -- but we have four combinations to work with. Calling the twins A and B, the combinations are:

A is Jon, A is the liar
A is Jon, B is the liar
B is Jon, A is the liar
B is Jon, B is the liar

Whatever question it is, it has to be open-ended.

Possible answers
Yes
Yes, but my name isn't Jon
No
No and my name isn't Jon

How about the answer "My name isn't Jon"

Have a Good Day
KJ Ramsey

You ask the twin a single question to derive 2 elements, identity and disposition towards the truth. No question can "force" them to address both elements.

It's not actually a sentence at all. Nor, formally, is it a question. If it were one sentence, that wouldn't mean it was one question: it's quite easy to formulate several questions in one sentence.

Moreover it doesn't work anyway. If you happen to ask the liar and he says 'No, and my name is Fred' - which is likely - where are you? You know he's the liar, and his name therefore is not Fred, but that doesn't prove it is or isn't Jon.

However, it is only one question. The way it is constructed, it can only reasonably be interpreted as being a question about the boys' status as twins which is addressed to Jon. If the person you are talking to is not called Jon, he may reasonably assume he is under no obligation to answer the question. A more likely response is "You've got the wrong person -- he's John", and then where would you be? Similarly, the sentence, "You are twins, Jon," is a single statement addressed to Jon, and is not the same as "You are twins, and your name is Jon."

For the question to work, we have to make a special assumption: specifically, that everyone has agreed beforehand that the twins must restrict themselves to those four responses. That may well be the case, but it's not stated in the riddle and so it is not a safe assumption.

Right. It would normally be used to mean the same as "Jon, is that you?" and as such would mean: I think you're Jon -- you look and sound like Jon -- but I'm not sure; can you please confirm that you are Jon?

In putting one question to two people you are actually in effect asking two questions.

"You twins, Jon?" is a single, 3 word question the answer of which indicates both the identity and the disposition towards the truth of the twins.

And, though both of the twins hear the question, you are in fact addressing it at only one of them, the one named Jon. You are "asking only one of the brothers", even if your question isn't "directed" explicitly at one of them by providing a physical reference to one of them( nodding or pointing to the one on the left, up, far etc.)

The two possible answers are "I am Jon" or "I am {some other name}". There are four possible situations, though, and they are:

1. You are addressing Jon, who always tells the truth. Answer: "I am Jon"

2. You are addressing Jon, who always lies. Answer: "I am {some other name}"

3. You are addressing the other, who always tells the truth. Answer: "I am {some other name}"

4. You are addressing the other, who always lies. Answer: "I am {some name which isn't his, and he might even claim to be Jon}".

So, if the person answers "I am Fred", the best you can do is to discount situation number 1. That still leaves three. If he answers "I am Jon", you can only discount possibilities 2 and 3, leaving 1 and 4 for you to guess at.

If the answer is "I'm Jon", you can't determine if
-he is Jon and he says truth
-or he is the lier and he is not Jon

If the answer is "I'm George", you don't know if
-he is Jon and he lies
-or he is the other and either says truth or he lies

Claiming to have any other name becomes irrelevant, as we have allready established the identity of the twin named Jon.