I had a boyfriend who had the theory that any story is allowed one great impossibility, but anything more than that starts to take you out of the story. I'm not sure I'd limit it to one, but I think the concept is accurate enough. Suspension of disbelief is easiest when you have to lift the least.
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Gillian
"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"
"You can't erase icing."
"I can't believe it doesn't work! I found it on the internet, man!"