What explains the change in the phases of the moon?

The change in the phases of the moon is caused by regularly occurring changes in the conditions of illumination of celestial bodies relative to the observer, when the moon moves along its orbit. The phase change occurs as a result of a change in the relative position of the Earth, the Moon and the Sun, since the Moon shines with light reflected from the Sun. The Moon is illuminated by the Sun along its path around the Earth, it does not itself shine.