Staples, somewhere along Rockville Pike, 5 p.m.-ish today...
Me to Clerk: Hi, I need to exchange this 12 x 12 x 12 shipping box for this 18 x 12 x 12 one. I miscalculated my shipping needs just slightly.
Clerk to Me: Ma'am, these boxes are the same.
Me: Uh... no, they're not. One is 12 x 12 x 12. The other is 18 x 12 x 12.
Clerk: They're both marked "medium," ma'am. They're the same.
Me: Nooo. They're different sizes. One is bigger than the other.
Clerk: No, ma'am. They are the same. It's just that one is a square and one is a rectangle. They just are shaped differently, so it's an optical illusion. They are both medium boxes.
Me: Umm... that's not an optical illusion. One is bigger. The one with the EIGHTEEN in the dimensions is bigger than the one with all twelves in the dimensions.
Clerk: No. (Points at writing on both boxes.) See? They are both marked MEDIUM so they are the same size. You can just keep your first box, and it will fit the same.
Me: You're kidding, right? Look, I may have flunked out of honors math in high school, but even I know that these boxes have different volume. There are several box sizes and shapes you carry - some are in the small range, some are medium, and some are large.
Clerk: And these are both medium, so they hold the same amount, but can hold different shapes.
(At this point, I start to assemble both boxes. I was the only customer up front. Man, I wish there had been an audience for this.)
Me: See? This box (points to 18-incher) is bigger. It also costs fifty cents more than the other one.
Clerk: Oh, it's more expensive? Then it must be bigger.
Me: (smiles) Yes. It is.