The moon has phases because the moon naturally does not glow. The light from the moon is actually the suns light being reflected off the moons surface. The moon has different phases depending on how much light is being reflected off it by the sun. The amount of sunlight reflected depends on the moons location compared to the sun and earth. As the moon goes around the earth it makes a full cycle.
The video below illustrates how this works:
This video is very usefully because it shows the process of the moon cycle in pictures. So it uses both visual and audio teachings to the topic. It is a very well explained video clip that is easy to comprehend and understand. It uses simple langue that anyone can understand.