It doesn't. We just see different slices of its sunlit half as it orbits Earth.
01 ·§ 01 · SECTION · 1/3
The moon is always half-lit
The moon makes no light of its own. The sun lights exactly half of it at any moment, the same way it lights half of Earth. That sunlit half never changes size. What changes is how much of it points toward us.
two weeks of orbit
the moon's lit half facing away from Earth
→
the moon's lit half facing toward Earth
When the lit half faces away, we see darkness (a new moon). When it faces us, we see a full moon. Everything between those is a phase, a partial view of the same lit hemisphere.
§ 02 · SECTION · 2/3
Phases are a question of geometry
the moon orbits Earth roughly every 29.5 days→the angle between sun, moon, and Earth keeps shiftingbecause the moon changes its position relative to the incoming sunlight
that angle decides which part of the lit half we can see→the visible shape grows and shrinksbecause we only ever see the overlap of 'lit' and 'facing us'
the cycle repeats→the same phases return in orderbecause the orbit is regular and predictable
Waxing means the lit portion is growing toward full; waning means it is shrinking back toward new. The word 'gibbous' just means more than half but not yet full.
§ 03 · DEEPER
The full cycle as states
Because the same side of the moon always faces us (it is tidally locked), the phases are never about the moon turning around. They are only about where it sits in its monthly lap of the Earth.