What is a cat in a box?

Cat in a box, I think they are like an upside down tortoise or turtle? Or maybe a hermit crab? Inhabiting just the right sized box…. Finding a box the right size they can pour themselves into. We have a small medium and large cat and each have tried this box. The big cat has to stick his head over the edge, the small one could fit in twice. The middle one is like Goldilocks, it’s just the right size for the box. Wiggle room to twist over for a belly rub. I sometimes tip a bit of catnip in…. Which is most appreciated…. OK… Life is good for our cats….