A. The Moon itself doesn’t change shape, it just appears to do so. It’s how much
of it we can see that actually changes.
The Moon is made mainly of rock and doesn’t produce any light itself. We only see
the Moon when the Sun is shining on it. The portion – or shape – of the Moon that
we see depends on where it is in its orbit around the Earth. Or, in other words,
it depends on its position relative to the Earth and the Sun.
The Moon orbits the Earth every 29.5 days. This is also the same amount of time it
takes the Moon to rotate once on its own axis meaning that the same side of the Moon
always ends up facing us down on Earth. When the Sun is setting behind the Moon (i.e.
when the Moon is between the Earth and the Sun), the side facing us is predominantly
dark and we can only see a small portion of it. As the Moon continues its orbit and
eventually reaches the opposite side of the Earth to where the Sun is, the entire
side of the Moon facing us becomes lit by the Sun and we can now see a full Moon.