The truthful (or sincere) and liars are a very common fauna in the world of riddles. They are also known as "see you and mentos", "knights and rascals", knights and squires "and in many other ways. Its main characteristic is that, for some kind of psychological impulse the truthful ones ALWAYS tell the truth and the liars ALWAYS lie.
On one occasion, I had the following conversation with Julio and Verne:
- I: - Julio, are you truthful or a liar? Unfortunately, the noise of a motorcycle did not let me hear his answer.
- Then I asked Verne: What did Julio say?
- Verne: - He said it was true.
Is Julio truthful or a liar?
Here we don't hear the answer to the first question, but it doesn't matter because we know from the liar's paradox that his answer was -I'm true. So Verne’s statement is true and the liar is Julio.