This weekend, Zac Vawter walked up some stairs. A lot of stairs.
103 flights to the top of Chicago's Willis Tower - one of the tallest skyscrapers in the world.
That alone is pretty impressive. But it's even more amazing because Vawter is an amputee, who lost his right leg in a motorcycle accident.
Vawter climbed all those stairs with a new, bionic, prosthetic leg that's controlled by his thoughts.
Vawter, who's 31, said "Everything went great. The prosthetic leg did its part, and I did my part."
Here's a video of the climb, that's posted on The Telegraph's website.
The leg responds to electrical impulses from the muscles in his hamstring. In other words, when Vawter's brain says "climb stairs", his leg effectively knows it and move the way a leg normally would.
The leg was developed at the Rehabilitation Institute of Chicago, which has done pioneering work creating bionic, thought-controlled arms.
But now, people who've lost a leg are outnumbering people who've lost arms and hands, so the institute is focusing more on lower limbs.
Vawter says the bionic leg responds better and moves more freely than his regular prosthetic. And as an engineer, he's enjoyed learning how it works.
To prepare for the climb, Vawter and the scientists spent hours adjusting the leg's movements. Along the way, he kicked a soccer ball, walked around the room and climbed stairs.
For Vawter, all of this started in 2009. When his leg was amputated, a surgeon took the nerves that carry signals to the lower leg and sewed them to new spots on his hamstring.
That opened the door for Vawter to one day to be able to use a bionic leg.
Levi Hargrove - a biomedical engineer from Bathurst, New Brunswick - has led the research and development of the leg. He works at the institute's Center for Bionic Medicine.
The surgery is called "targeted muscle reinnervation" and it's like "rewiring the patient," Hargrove says. "Now, when he just thinks about moving his ankle, his hamstring moves and we're able to tell the prosthesis how to move appropriately."
Hargrove said his inspiration came from Terry Fox, and his Marathon of Hope in 1980.
"I've run marathons, and when you're in pain, you just think about Terry Fox who did it with a wooden leg and made it halfway across Canada before cancer returned," he says.
The leg is still years away from being available on the market but Vawter says "Somewhere down the road, it will benefit me and I hope it will benefit a lot of other people as well."
Along with Vawter, nearly 3000 people took part in this weekend's climb, called SkyRise Chicago, to raise money for the institute.