When I was a young child, my family and I traveled to Chicago.  We went there because my dad was completing a stair climbing race in the Willis Tower.  At the time, the building was known as the Sears Tower.  While at the tower, we visited the skydeck.  The skydeck is an observatory at 103 stories above the ground.  There is a box you can walk out into which hangs over the street below.  This entire box, including the floor, is made of glass (or at least a see-through material) which makes for an exciting time.  The sheer height of this building is amazing to see in person, especially from 103 stories looking down.  All the people and cars look like little dots or ants on the ground.  This building is a must see for anyone visiting Chicago.