Why does moon change its shape?
Moon does not change its shape, but we see only part of the moon which makes us think that moon changes its shape. Moon does not have light of its own. It reflects light from the sun. We only see the part that reflects light. Moon goes through phases for about 30 days where its shape changes and the cycle repeats. The different phases of the moon are New Moon, Waxing Crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, waning crescent and then new moon again . New  moon is when no moon is seen in the sky. Slowing we start seeing part of the moon. When we see crescent shape after new moon it is called waxing crescent. First quarter is when we see moon as a semi circle. We continue to see more part of the moon towards waxing gibbous and then eventually we see complete circle on Full Moon. After this, some part of the moon starts being hidden and unable to see. This result in waning gibbous. Soon we again see semi circle of the moon known as Third quarter. Parts of moon continue to get hidden and we see only a crescent shape known as waning crescent and back to New moon.