Some things are designed so that when they are perfectly in place, they will "click" into place. The most recent thing that matches this was putting my camera's memory card back into place -- as it slid into place, it literally clicked.
We talk about thinkings "clicking" when they make sense to you. It fits into a logical place in your head.
That's the metaphorical click here. It feels so right that you know it's in the right place.
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