Observe the moon continuously for several nights, preferably from one full moon to the next. Make a sketch of the moon every night in your notebook and note the day from the day of the full moon. Also, note every day the part of the sky (east or west) in which the moon is seen.
We find that there is a change in the shape of the moon every day and the moon appears to be perfectly round on the full moon day. On the fifteenth day of the month, we see that the moon is not visible even if the sky is clear.
Thus, the moon goes on increasing every day, till on the fifteenth day and the full face of the moon is visible. After the full moon the bright part of the moon goes on decreasing every night and by another fifteen days again new moon is formed.