The Simple Question

Two twins, Juan and Pedro. One always lies, the other always tells the truth But you don't know which one. You find one of them on the street. With one question, with a "yes" or "no" answer, you have to guess his name. What is the question?

The question to ask the twin you meet is:

"Is Pedro the liar?"

If he answers "Yes" he is Juan. If he answers "No" he is Pedro.

To see why, imagine asking the question to each twin if he is the liar and also if he tells the truth. For example if the twin you meet is Pedro and he is the liar, what would he reply?