The Gimme

As in "just give me this one departure from fact, and I'll spin you a tale that will blow your mind." It's a staple of all fiction, but it crops up the most in F/SF.

There is an economy of the Gimme. Don't use one more than once a story. It should have ramifications. It should generate surprises. It should allow for wonder, shock, recognition, laughter, sorrow.