Not just any paper sack, mind you. I have in mind:
the big boxy paper sack from the grocery store,
with a rectangular bottom, that folds just so.
An empty, unfolded paper sack, placed open on the floor
with the boxy opening pointing horizontally toward the cat,
as if to say:
"why don't you creep inside and make yourself comfortable?"

When I was a child, and watched the pet cat
creep out of sight into the depths of the paper bag,
I foolishly felt deserted -- why won't the cat come back out and play?


Is it the privacy of a space just large enough for a one-cat den?
Is it a territorial thing: this space is MINE now, says the cat.

Why are pet cats fascinated with a paper sack?