The phases of the Moon happen because the Moon is always half lit by the Sun, but we do not always see the same half from Earth. As the Moon orbits our planet, the angle between the Sun, Earth, and Moon changes, so the visible portion of the lit side appears to grow and shrink.
That is why the Moon can move through familiar stages like crescent, quarter, gibbous, and full. The Moon itself is not changing shape. Our view of the sunlit half is changing as the orbit progresses.
The cycle takes about a month because the Moon needs roughly that long to complete its orbit around Earth. The exact timing can vary a little, but the pattern is reliable enough that people have used it for calendars and navigation for a very long time.
If you have ever wondered why the Moon looks different on different nights, the answer is geometry, not mood or weather. The same object is being illuminated from the same direction; we just see a different slice of it from where we stand.
So the short version is simple: the Moon’s phases come from its orbit around Earth and the changing way sunlight hits the part we can see.
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