Help kids understand this concept by taking them on a shadow hunt on a sunny day. In each case, let students spot the shadow first and then search for what's casting it. Point out how shadows also give objects shape and texture by highlighting edges and contrasting irregularities in surfaces. Point out that in ancient times some people believed that their shadows were a separate, special part of themselves. The size of a shadow depends on the way the light rays strike the object.
Light travels in a straight line and a shadow ends where the light rays clear the top of the object.
Early in the morning and late in the afternoon, when light strikes objects at an angle, shadows stretch long and skinny. At noon, with the sun almost directly overhead, light rays are nearly vertical and shadows are short and fat. Demonstrate this for students by setting an object, like a brick, on a light-colored cement sidewalk on a sunny day. Every hour, have different groups of students measure the length of the brick's shadow and outline it with a different colored chalk. Back in class, have each group add a bar to a graph, showing the shadow's length and how it is changing from hour to hour.