Distance and Depth

When we look at two objects we can say which one is further away.
When drawing we can use two principles or rules:

The principle of SUPERPOSITION :

When an object is in front of other object it hides part of the object behind.

In this picture the purple circle is in front of the green one. So the purple circle hides part of the green one.

The principle of SIZE:

When an object is in front of other object it looks bigger than the object behind.

In this picture the orange circle is in front of the blue one. So the orange circle looks bigger.

In this picture you can see three giraffes. 

There is a giraffe at the background, a giraffe at the front and a third giraffe in between.