Imagine that the moon isn’t tidally locked. As it rotates by, side of the moon facing the earth feels a gravitational pull towards it, and as it rotates away the gravitational pull acts to slow it down just a tiny bit. Over time this “gravitational friction” slows the moons rotation until it becomes tidally locked, where the same face of the moon is always facing earth. This is a stable condition because if a force were to try to change the rotation speed of the moon in either direction, than the gravitational friction will come into play again.
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