The moon has phases because it orbits the Earth and the sunlight illuminates different portions of the moon's visible surface from our perspective on Earth. As the moon moves in its orbit, the angle between the Earth, moon, and sun changes, causing the illuminated part we see to vary, creating phases like new moon, first quarter, full moon, and last quarter. The moon itself does not emit any light; it reflects sunlight, and the changing view of the lit portion is what causes the phases.