At the beginning of the month, the moon is thin and curved. Gradually become fat. A week later it became a crescent moon. Two weeks later there is a full moon. Then it starts to thin again. After three weeks there is a half moon. Towards the fourth week, there is no new moon. Then the month begins again through the curved crescent moon. We see the moon as bright as the moon shines from the sun. So the same moon takes on a different shape due to the increase or decrease of light receiving. This process continues due to the rotation and position of the earth and the moon.