Okay guys, obviously the first 25 conundrums were easy, with the exception of 13, to which you still have no answer. So, while you guys are criticizing the answers for those last one's, try your luck at this:

An astronaut lands on the planet Zebor and is confronted by two identical aliens : one who always tells the truth and the other one who always tells lies. There are two doors, one leading to a cave of unlimited riches and the other to a giant human eating plant. He is allowed to ask one of the aliens one question to choose the right door. What should he ask?