There's a charming children's book series called "the Great Brain," where a boy in the early 1900s recounts the amazing mental feats of his unusual older brother.
The older brother had a strategy for the tough problems: he'd think about the question just before going to sleep, to engage his subconscious. It always worked for him. It's often worked for me, too.
Step away from the problem. Walk to the vending machine, to the parking lot, to the corner market, or just sit on the couch at home for a little while. No paper, no pen, no books. Just let your brain cruise.
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