You wake up. Quite jarringly, in fact. Your head hurts, and you seem to be in a polished metal box. There's a set of doors on one side, but no doorknob. Right next to the door, there's a small panel. It has half a dozen buttons, labeled 1-6.

The button with the number 6 looks as if it's out of order. The button is hanging out of the socket by a red wire.

You're sure you've been in a place like this before, but you seem to remember it moving. Yes, these things are supposed to move. This one must be stuck. Call for help? Yes, you can call for help. But... how? You have no way to contact any people, and the elevator has nothing but the doors and the buttons.

Maybe you should press one.

You press the button labelled "4", and... nothing happens.

Well, that was anticlimactic. Is this the floor you're on?

But the doors remain closed.

Now what?

Maybe you didn't press it hard enough. Or you didn't press it quite the right way.

You try again—harder this time. Still, the doors refuse to budge.

And again. Maybe holding it would work? Third time's the charm after all.

Still nothing.

There's five—erm... four other perfectly good buttons you could be pressing right about now. Why did you pick this one?

Perhaps it's because four is your lucky number.

Perhaps it's because it's unlucky.

Whatever your reason may be, you fail to notice yourself losing balance during your futile attempts to convince button 4 to do something. And your other hand slips and presses another button at the same time! This elevator lets you do that? Before you could ponder over that question, the elevator starts shaking violently—unsure of which floor to stop at.

It seems pressing button 4 and another button simultaneously had an entirely different effect on the elevator than if you were to press either button individually. But what was that other button you had pressed?