A man enters a department store.

- Hello, I'd like to pick up some moisturiser.

- Certainly, what color would you like?

- I umh... sorry, I don't follow. Do you mean what color would I like my skin to be after using it?

- No no no, what color would you like the moisturizer itself to be?

- I guess I haven't given it much thought. I suppose any color will do, as long as it doesn't disturb the color of my skin. I rather like having all my skin be the same color.

- Oh rest assured, it doesn't, perfectly safe to use.

- Great, so can you give it to me?

- Actually that's not how we do things around here. I will give you this order form and you'll have to go pick it up.

- Whereabouts?

- On the fifth floor, take the elevator.

- Great, thanks.

The man exist the elevator.

- Hi, I have this order form, I need to pick up some moisturizer.

- I'm afraid you're on the wrong floor, that's one floor down.

- Isn't this the fifth?

- It's the sixth.

- But I took the elevator and I pressed +5.

- And where did you start off?

- On the ground floor.

- Oh, that's a common mistake. There is no ground floor.

- Okay, the one at street level.

- That's the first floor.

- Isn't it called the ground floor?

- Why would it be? It's the first floor above the ground, so it's the first floor.

- But some people call that the ground floor.

- Look, have you ever counted thermometers?

- Why thermometers?

- Nevermind, just answer the question.

- I have.

- And how many did you have in all?

- Three.

- And which one was the ground thermometer?

- So one floor down?

- One floor down.

The man exits the elevator anew.

- Is this the fifth floor?

- The fourth.

- But but. I was just on the sixth and I pressed -1.

- I guess you must have been on the fifth then.

- But the guy up there told me it was the fifth.

- Maybe he miscounted.

- That's what I said! Anyway, so how do I get to the fifth?

- +1.

- Thanks.

The man exits the elevator on a new floor.

- Is this the fifth floor?

- The sixth.

- What?? But I've been to the sixth already and the guy that was there is not you, this can't be sixth.

- Oh, that doesn't necessarily mean anything. The staff here rotate frequently, he could be on break.

- Okay look, I was just on the fourth and I pressed +1.

- You know, some elevators only run in two floor increments.

- But there's only one elevator.

- It looks that way, but actually there are several.

- But how can that be? They would collide with each other.

- Oh, they don't run on the same line. It's just that you can't tell from inside the elevator, because they built it in such a way as to model the traditional upwards and downwards sensation of movement so people wouldn't freak out.

- But if I pressed +1 shouldn't it go one floor up?

- The number is multiplied by the increment, so if it's one of those that only move in two floor increments, you'd be going 2 up.

- So how do I know what the increment is in the elevator that I'm in?

- Oh, that's a bug. They were supposed to have a panel to display that, but by the time it was built they noticed they forgot it.

- So how can you ever be sure which floor you're on?

- You develop a certain sixth sense when you work here.

- And if you don't work here?

- Well, you can always ask us.

- Okay, so how do I get to the fifth?

- Here's a tip. If you're in the by-two-floors elevator you can just type in -0.5. And if it turns out to be the one that moves in single floor increments, it will round up to -1 anyway.

The man exits the elevator yet again.

- You again! But you're the guy I spoke to originally, the guy who gave me this form!

- Welcome back.

- But I was just on the sixth floor, I typed in -0.5 to get to the fifth to pick up my blasted moisturizer and I come down all the way here!

- That's a simple misunderstanding. You see, some elevators don't work in increments, they work in absolute values.

- So.. wait, if that's the case and I typed in -0.5 then it should round up to -1.

- No, it rounds up to 0. Otherwise it would be rounding down.

- So what floor is this?

- The zeroth.