Why can we sometimes only see only part of the moon lit up?

We can sometimes see only part of the Moon lit up due to its phases, which are determined by the relative positions of the Earth, Moon, and Sun. As the Moon orbits Earth, different portions of its surface are illuminated by sunlight, resulting in phases such as new moon, crescent, first quarter, gibbous, and full moon. The angle at which sunlight strikes the Moon also affects how much of it appears lit from our perspective on Earth.