tail in its mouth."

“Ourobouros,” I said. “At least I think that’s its name.”

“Whatever,” she said. “Anyway, you punch in the numbers here, and then push this button.” It was large, and blue, and beneath it, in great bold letters, was a sign: NO PARADOXES PLEASE.

“I think I get it. What happens if I set the t coordinate to, I don’t know, maybe five, ten minutes ago?”

“Never tried it,” she said. “I’ve only been to Paris on Liberation Day.” She flashed me a flirty little smile. “Why not, though? I don’t think there’s any higher energy expenditure to go back five minutes than a hundred years.” She started typing, her fingers nimble on the keys.

“You’d think it’d be less,” I said.

“You’d think,” she said, “but the professor says it doesn’t work that way. It’s an asymptotic energy curve, he says, a wall that the universe has thrown up against time travel. But he’s found a way to tunnel through.”

“It needs a name,” I said.

“Yeah,” she said, pushing the blue button, "maybe like that snake that has its own