Riddle of the day

Here is the answer:
The kid did it because the school isn't open on Saturdays
And here is the next part of the riddle:
There are two identical doors, but only one has a treasure inside. Each is guarded by a robot, "X" & "Y". The doors and robots are identical in every way, except one robot always lies and one always tells the truth. But you don't know which one is the liar.
What one question, could you ask only one robot, and know which door to open to find the treasure?