What happens to the moon after a full moon day?

Hint: We use the lunar cycle and the moon phases to answer this question and explain the scientific reason behind the cause. Every night after a full moon day, the size of the moon keeps decreasing. The visible part of the moon keeps becoming smaller until the 15th day. On the 15th day the moon is completely invisible this is because no light falls on the moon for it to be visible. This day is called a new day. 15 days later from the new moon day another full moon arises.

Complete answer:
The moon is a natural satellite of the earth. It rotates around the earth like the way the earth rotates around the sun. It takes around 27 days for the moon to return to its position on its axis from the position it starts revolving.
The moon is non-luminous. By which we can say that the moon is visible to us only due to the sunlight reflecting from its surface to earth. While the moon is revolving around the earth the amount of sunlight that falls on the moon to get reflected is different every day based on its position. At different positions of the moon the earth blocks part of the sunlight falling on it hence we see different phases of the moon.
This way different phases of the moon are seen. A full moon day is when the complete moon is visible. After a full moon day, the size of the visible part of the moon keeps getting smaller. On the 15th day, the moon is completely invisible and this day is called the new moon day. 15 days from the new moon day another full moon day arises. From one full moon day to another full moon day it takes approximately 27 days.

Note: The moon is always in its axis, during the day as well as night. But it is only visible during the night because the reflected light intensity is low compared to that of sunlight. In the day due to the presence of sunlight, the reflected light from the moon is not visible. Hence it is visible only during the night.