Re: What property of light allows shadows to form?

First, I'll do as I always do and answer it quickly: the property of 
being able to be physically blocked.  How?  The answer is simpler than you 
think.  Thinking of shadows as existing entities is like thinking of cold 
as an existing entity.  A shadow is merely the abscence of light, as cold 
is the abcense of heat.  Now, there's a lot of crazy stuff about lights 
duality -how it's both a particle and a wave-, but this doesn't change the 
fact that it can be blocked.  
     So, when you shine light on an object, the light is blocked by the 
object, being reflected and absorbed.  If you look behind the object in 
the direction of the light's path, then you see an area in the shape of 
the object that blocks the light that's darker.  This is the shadow.  Of 
course, this area isn't in COMPLETE darkness, there's always light 
bouncing around from all sorts of different sources, and perfect darkness 
is rare.  So, basically, shadows are just the areas where light isn't. 
What interesting is where the light IS...
    This question was answered on the simplest level possible.  If you are 
interested in learning more about light, send more questions!