"So when this girl laughed, I said, 'I'll bet I write one and sell it,' and she said, 'I'll bet you don't,' and I said, 'I'll bet you a date against five dollars. If I sell the story, you go with me to a dinner and dance on a night of my choosing.' And she agreed.

"So I've just got to write the story now, because she said she'd go out with me if I wrote the story and she liked it, even if it didn't sell-which may mean she likes me more than I think."

James Drake, who had been listening thoughtfully, brushed his gray stub of a mustache with one finger and said, "Or that she's quite confident that you won't even write the story."

"I will," said Peterborough.

"Then go ahead," said Rubin.

"There's a catch. I can write the story, I know. I've got some good stuff. I even know the ending so I can give it that backward look you mentioned, Mr. Rubin. What I don't have is a motive."

"A motive?" said Rubin. "I thought you were writing a science fiction story."

"Yes, Mr. Rubin, but it's a science fiction mystery, and I need a motive. I have the modus operandi of the killing, and the way of killing but I don't know the why of the killing. I thought, though, if I came here, I could discuss it with you."

"You could what?" said Rubin, lifting his head.

"Especially you, Mr. Rubin. I've read your mystery stories-I don't read science fiction exclusively-and I think they're great. You're always so good with motivation. I thought you could help me out."

Rubin was breathing hard and gave every appearance of believing that that breath was flame. He had made his dinner very much out of rice and salad, plus, out of sheer famishing, two helpings of coupe aux marrons; and he was in no mood for even such sweet reason as he was, on occasion, observed to possess.