"Because a liar wouldn't be a liar if he spoke the truth. Given the condition isn't met, his answer could be either true or false. No matter what he'd answer, there's a chance he'd tell the truth."

The condition not being met does not magically change the facts, though. Suppose;
A=T B=T C=L D=L

Ask D "if A and B are different, is A a truth-teller?"

A and B are not different, but asking the question does not change who is a liar and who tells the truth. If D replies 'no' it is still a lie.
If you want to say the condition then makes it a hypothetical situation different from reality if the condition is not met, then what is to prevent the question from applying to a hypothetical situation differing from reality when it is met?


In the case of being asked if someone is holding up two fingers behind their back, you do not know the truth, so can not definitely answer yes or no (information the asker already had), and the inability to answer is not an answer.
In the riddle, the inability to answer the conditional question truthfully answers the question 'are A and B different?'.

Speaking of which, lets say A and B both tell the truth, but A is a male 250lb football player and B is a female 90lb office manager. Now you ask D 'If A and B are different...' .....


"But the answer is different (if asked seperately), how can the question be the same?"

Functionally, you can phrase two separate questions to get the exact same information.


"I don't feel teased yet <img src="/ubbthreads/images/graemlins/tongue.gif" alt="" /> Or wrong <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" /> But do keep trying! <img src="/ubbthreads/images/graemlins/cool.gif" alt="" /> "

The reason you didn't realize you were being teased is because you are still in a delusional state believing you are right about the riddle.
<img src="/ubbthreads/images/graemlins/silly.gif" alt="" />



The helicopter riddle;
[color:"#3A495A"]The information was 100% accurate, yet completely useless, so the pilot knew he was outside Microsoft's support building, and thus where the airport was relative to his current location.[/color] <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />