Chicago is home to one of the tallest buildings in the world: the Willis Tower. So tall that on a windy day, the topmost floors can sway as much as three feet. But what keeps it from falling?

Reservations will be required from 8 to 10:30 a.m. daily at Sand Harbor starting April 15 through Oct. 15. Same-day bookings are allowed.

Landmark opened in 1973 with original tenant Sears Roebuck & Co. CHICAGO, Illinois (CNN) -- Sears Tower is history. As of Thursday, the iconic Chicago, Illinois, skyscraper is now named Willis Tower.