Shadows are created when an object, animal or person blocks out a portion of a light source. Shadows form in the opposite direction of whatever is blocking that source. For example, if the sun is shining to the right of someone, a shadow appears on that person's left side.
Clouds and planets can also create shadows. Clouds create shadows whenever they move in front of the sun. During the evening hours, the sky becomes dark when the earth moves away from the sun and blocks its rays from entering the sky. Trees and buildings that continuously block out sunlight create shadows that allow for permanent shade.