alt.total-loser

It's the inverse of Chekov's Gun ("If there's a gun on the table in act 1, it should go off in act 3" or someone like that). Asimov's Tail is like saying "If a gun goes off in act 3, it should be on the table in act 1."

You don't need that for guns because we all know what guns are. A character in a modern setting pulling out a gun to win a fight on page 120 isn't going to bother the reader. But a tail is unexpected: You need the reader to buy in to and expect the tail before it becomes plot-relevant, or it'll read like a deus ex machina.

(Another comparison is Chekov was talking about the stage, and Asimov is talking about books. Asimov's Tail may not be as applicable to the stage, where the audience can all clearly see the character's tail in his costume.)

“And we loved it, we were all having a great time, but then he saw my sister and I, and he just got this - this look in his eyes, and then-”

She hesitated, worrying the coarse material between her fingers. “Things got pretty bad after that,” she muttered. “I know it’s silly, but I try to keep - her - comfortable. We don’t know if she can still hear us, or see us, or if she’s even still in here, but I like to think she is. I talk to her when I can, I leave music on when I’m out of the house. I tried to convince my parents to bring her with us when we went to Disneyland, but they didn’t - didn’t really take that well.”