As the moon travel around the earth,it appears to change shape. We associate certain names with certain shapes of the moon. For example, when we can see a small part of the moon,it is called a er moon. A crescent is a shape that looks like the tip of a fingernails. When we cannot see the moon at all,it's called a new moon. When we can see the entire moon, it's called a full moon. Usually,there is only one full moon every month. Sometimes,however,there will be two full moons in one month. When this happens,the second full moon is called a"blue moon.