Shadow

The thing about it is... what is it? I mean, we can classify things into solid, liquid or gas... but a shadow's not solid (It changes shape, doesn't it? Can't really measure the surface area very properly.)... it can't be a liquid because you can't measure it in terms of volume...and it can't be a gas because it's not really in the air around us...

So... what is it? Comments appreciated. :)