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Zac Vawter, fitted with an experimental "bionic" leg, looks out from the Ledge at the Willis Tower, Thursday, Oct. 25, 2012 in Chicago. Vawter is training for the world's tallest stair-climbing event where he'll attempt to climb 103 flights to the top of theWillis Tower using the new prosthesis.  (AP Photo/Brian Kersey) Brian Kersey / AP Photo