Good Omens by Terry Pratchett, Neil Gaiman

about their young. Whatever." He pulled himself together. "The point is. The point is. Their brains."
    He reached for a bottle.
    "What about their brains?" said the angel.
    "Big brains. That's my point. Size of. Size of. Size of damn big brains. And then there's the whales. Brain city, take it from me. Whole damn sea full of brains."
    "Kraken," said Aziraphale, staring moodily into his glass.
    Crowley gave him the long cool look of someone who has just had a girder dropped in front of his train of thought.
    "Uh?"
    "Great big bugger," said Aziraphale. "Sleepeth beneath the thunders of the upper deep. Under loads of huge and unnumbered polypol.. polipo.. bloody great seaweeds, you know. Supposed to rise to the surface right at the end, when the sea boils."
    "Yeah?"
    "Fact."
    "There you are, then," said Crowley, sitting back. "Whole sea bubbling, poor old dolphins so much seafood gumbo, no one giving a damn. Same with gorillas. Whoops, they say, sky gone all red, stars crashing to ground, what they putting in the bananas these days? And then.. "
    "They make nests, you know, gorillas," said the angel, pouring another drink and managing to hit the glass on the third go.
    "Nah."
    "God's truth. Saw a film. Nests."
    "That's birds," said Crowley.
    "Nests," insisted Aziraphale.
    Crowley decided not to argue the point.
    "There you are then," he said. "All creatures great and smoke. I mean small. Great and small. Lot of them with brains. And then, bazamm."
    "But you're part of it," said Aziraphale. "You tempt people. You're good at it."
    Crowley thumped his glass on the table. "That's different. They don't have to say yes. That the ineffable bit, right? Your side made it up. You've got to keep testing people. But not to destruction."
    "All right. All right. I don't like it any more than you, but I told you. I can't disod.. disoy.. not do what I'm told. 'M a'nangel."
    "There's no theaters in Heaven," said Crowley. "And very few films."
    "Don't you try to tempt me, " said Aziraphale wretchedly. "I know you, you old serpent."
    "Just you think about it," said Crowley relentlessly. "You know what eternity is? You know what eternity is? I mean, d'you know what eternity is? There's this big mountain, see, a mile high, at the end of the universe, and once every thousand years there's this little bird.. "
    "What little bird?" said Aziraphale suspiciously.
    "This little bird I'm talking about. And every thousand years.. "
    "The same bird every thousand years?"
    Crowley hesitated. "Yeah," he said.
    "Bloody ancient bird, then."
    "Okay. And every thousand years this bird flies.. "
    ".. limps.. "
    "flies all the way to this mountain and sharpens its beak.. "
    "Hold on. You can't do that. Between here and the end of the universe there's loads of.. " The angel waved a hand expansively, if a little unsteadily. "Loads of buggerall, dear boy."
    "But it gets there anyway," Crowley persevered.
    "How?"
    "It doesn't matter!"
    "It could use a space ship," said the angel.
    Crowley subsided a bit. "Yeah," he said. "If you like. Anyway, this bird.. "
    "Only it is the end of the universe we're talking about," said Aziraphale. "So it'd have to be one of those space ships where your descendants are the ones who get out at the other end. You have to tell your descendants, you say, When you get to the Mountain, you've got to.. " He hesitated. "What have they got to do?"
    "Sharpen its beak on the mountain," said Crowley. "And then it flies back.. "
    ".. in the space ship.. "
    "And after a thousand years it goes and does it all again," said Crowley quickly.
    There was a moment of drunken silence,
    "Seems a lot of effort just to sharpen a beak," mused Aziraphale.
    "Listen," said Crowley urgently, "the point is that when the bird has worn the mountain down