Why do people say to think outside the box?

We weren't thinking inside boxes in the first place. We could try thinking in the material that separates the inside of the box from the outside of the box, though. That's a unique alternative.

Or, as I have recently, thinking inside the Nuke, or outside the Nuke. I could also try thinking inside and outside of things... AT THE SAME TIME. Or would my head explode? You tell me, people who think things, I've never tried to think inside and outside of things at the same time, so tell me.

And yes, I know how to think inside the chimney, a rare skill, one to be proud of.