As you sleep and dream, you witness the universe shifting variables, altering rules, juxtaposing its parts in a sort of perpetual ontological shuffle button. When you wake up and find the world as you left it, it is only because the universe still hasn't found another shuffled state that fits its criteria. But given enough time, who knows? You may fall asleep in one existence and wake up in quite a different one.
That's what I'll tell my kids, anyway.