The Last Question at the Gate

You stand in a stone chamber with two closed doors. One door leads safely outside; the other opens onto a fatal trap. A guard stands beside each door. Both guards know which exit is safe. One guard always speaks truthfully, while the other always lies, but nothing about their appearance reveals which is which. You may address either guard and ask a single question that can be answered by indicating one of the two doors. After receiving the answer, you must immediately choose a door. What is the fewest number of questions needed to guarantee your escape, and what should you ask?