Where Do Dreams Come From?

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When we close our eyes and drift off to sleep, something in our mind spins us fanciful tales of teeth falling out, bouncing around in giant marshmallows in the sky, taking a walk in the park down the street that's also a spaceship. Common as they are, there's not a lot of definitive science on how we dream.
Are dreams the work of the imagination, or the work of some reflex in the brain?
It's a question that has long fascinated and flummoxed those who study human behavior.
Where do dreams come from?