Veritas and Truth are sisters. One of them (we don’t have any idea who) always tells the truth while the other always lies. Alex questioned one of them:
“Is Truth the one that lies?”
“Yes,” she replied.
Did Alex speak to Veritas or Truth?
Before I present the solution, think about this: Would someone who either always tells the truth or always lies call himself a liar?
The answer is obviously no. If someone always tells the truth, he would say that he is not a liar. Similarly, a person who constantly lies would also deny that he is a liar.
This is the basis for our solution. We can now conclude that Alex spoke to Veritas. Why? As stated above, neither the constant truth-teller nor constant liar would say that she is a liar. We know that one of the sisters is always telling the truth and the other is always telling lies. This means that when Alex asked whether Truth was the one who always lies, only Veritas could answer affirmatively.
Anyway, we still don’t know who among the siblings is the liar. Just for fun, can you think of one question (you can only ask one of them) that would enable you to determine which of them is the liar?