What causes the phases of the Moon?

The phases of the Moon are due to the movement of the Moon
around the Earth. As the Moon revolves around the Earth different phases are seen. One
important thing to note here is that for the phases to occur the Earth does not have to
be in the path of the Moon and the Sun. That happens very rarely and only during lunar
eclipses.

As the Moon revolves around the Earth, one half
of the Moon is always illuminated by light from the Sun. But whether we see that side or
not is dependent on the position of the Moon with respect to the
Earth.

You can see more clearly in the link provided below
the phases of the Moon and how they are dependent on where the Moon is. We see the Full
Moon phase when the Moon is positioned such that we can the side that is illuminated by
the Sun completely. On the other hand when the side of the Moon on which the Sun is
shining is facing away from the Earth, we have the New Moon Phase. As the Moon revolves,
intermediate stages between full moon phase and the no moon phase are
seen.

The phases of the Moon change every day and a
complete cycle takes approximately 29 and a half days.