You remember the Holy Grail scene from Indiana Jones? The problem is not the absence of the grail, somewhere on the shelf. The problem is all the others and the

You remember the Holy Grail scene from Indiana Jones? The problem is not the absence of the grail, somewhere on the shelf. The problem is all the others and the fact that if you pick the wrong one, your program desiccates, crumbles to dust, and is blown away by a conveniently dramatic wind. A grail shelf that wasn't intended as a booby trap would contain one cup, the right one.