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Zac Vawter practices walking with an experimental "bionic" leg at the Rehabilitation Institute of Chicago, Thursday, Oct. 25, 2012 in Chicago. Vawter will put his bionic leg to the ultimate test Sunday, Nov. 4, when he attempts to climb 103 flights of stairs to the top of Chicago's Willis Tower, one of the world's tallest skyscrapers. If all goes well, he'll make history with the bionic leg's public debut. (AP Photo/Brian Kersey)