Pretty Cool - an iron atom magnified 165 billion times!  It was part of the 1958 World's Fair that was held here in Brussels and we just had to go and see it.  You do get to go up inside of it - there is even one sphere that you can book for a children's overnight party.  There are escalators and stairs that take you between the levels within a sphere and thru the connecting rods between the spheres there are also escalators.  To get all the way to the top sphere you have to take the high speed elevator - up there is a restaurant (we didn't eat there) and an observation area (we did look all around that!).  It is an architectural feat, which is what drew us to go and see it!