It seems Watanuki’s reading comprehension is a trifle lacking.
He often leaves part of his body sticking out of the condo, but it’s usually a paw, or even his entire front half.
This is the first time I’ve ever seen him leave just his tail behind.
Yes, OK. His tail is always behind him. I get it. Very clever.
As best I can tell, he’s trying to emulate the Cheshire Cat, but he’s gotten the quote quite the wrong way around. “…this time it vanished quite slowly, beginning with the end of the tail, and ending with the grin, which remained some time after the rest of it had gone.”
“Well! I sometimes see a cat without a tail,” thought Alice; “but a tail without a cat! It’s the most curious thing I ever saw in my life!” Setting aside her saw, Alice continued down the path. “I do believe the March Hare will require an ax.”
—Alice II: Nightmare in Wonderland