Why do shadows form?

All of us have seen our shadows when we go out in the sun. In fact, all objects including trees and houses have a shadow when they  are in sunlight. Why does this happen? Sometimes, objects are able to block light. An object through which no light can pass is called opaque.
An example is our body. Since light cannot pass through, it creates a dark area around the object. this patch where no light falls, it called shadow.

A shadow moves because the light rays keep changing their direction in which they fall on the object. if the object moves, then again the shadow moves, as it formed whereever the object blocks light.

Shadows are longest in the early morning and late evening, and shortest at noon, because of the angle at which sunlight falls on an object.