Same ball, different view
Why the Moon Changes Shape
The Moon never changes shape. You just see different amounts of its lit side.
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Step 1 of 5
The Moon makes no light of its own.
It is a grey rock. Everything you see of it is sunlight bouncing off its surface toward your eyes.
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Step 2 of 5
Exactly half of it is lit, always.
The Sun lights one side of the ball, all the time, without pause. There is no moment when the whole Moon is dark or the whole Moon is lit.
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Step 3 of 5
The Moon circles Earth once a month.
As it goes round, your viewing angle onto that lit half keeps changing. Sometimes you look at it head on, sometimes from the side, sometimes from behind.
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Step 4 of 5
So the shape is really an angle.
A crescent is you seeing the lit half almost edge-on. A full moon is you seeing it face-on. Nothing about the Moon changed — your seat did.
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Step 5 of 5
It takes 29.5 days to run the loop.
That is where the idea of a month comes from — the word month is a worn-down version of moon.
The Moon always shows Earth the same face. Its spin and its orbit take exactly the same time.
The short version
Exactly half the Moon is lit at every moment; the shape you see is the angle you see that half from. One lap takes 29.5 days, which is where the month comes from — the word is a worn-down moon.
Try it yourself
Hold an orange at arm's length in a dark room with one lamp on. Turn slowly in place and watch the orange run through every phase.