guard

[massive1]

2 guards, 2 doors,
one guard tells iies
one guard tells the truth.
one door is life
one door is death
you can only ask one guard one question
which room is the door of life?

[squarebear]

He asks either of the guards:

"If I ask the other guard which is the door of life what will he say"?

He then chooses the opposite door to the one he is told.

[plowter]

If you like logic puzzles like this, try Raymond Smullyan's classic puzzle book 'What Is the Name of This Book?'