tidal locking
The Moon always shows us the same face — we have never seen its far side from Earth without sending a spacecraft. That is not a coincidence: the Moon's spin and its orbit are perfectly synchronized, so it turns exactly once on its axis for every trip around the Earth. This locked-in rotation is called tidal locking.
It happens because tidal forces raise a slight bulge on a moon, and if the moon spins faster than it orbits, that bulge is dragged ahead of the line to the planet. The planet's gravity tugs back on the leading bulge, applying a gentle braking torque that slows the spin — until spin and orbit match and the bulge sits steadily on the planet-facing line, with no more twisting. The same friction-like process is why most large moons in the Solar System are tidally locked to their planets, and why the planet's own day is slowly lengthening too.
Tidal locking shapes worlds. A tidally locked planet around a small red dwarf star would have a permanent day side and night side, with profound consequences for its climate and any chance of life. In the most extreme cases two bodies become mutually locked, each forever showing the same face to the other — as Pluto and Charon do — turning a pair of worlds into a single rigid-looking dance.
Mercury is not fully locked to the Sun but caught in a 3:2 spin-orbit resonance: it turns exactly three times on its axis for every two trips around the Sun, a subtler relative of tidal locking forced by its eccentric orbit.
Locking need not be 1:1 — eccentric orbits can trap other whole-number spin-orbit ratios.
A tidally locked body is not motionless: it still rotates, just once per orbit, so 'the Moon does not spin' is a common error. And a locked planet's far side is dark only if it is locked to its star, not because it never receives starlight by some other rule.