Peak performer.

Last summer, 20-year-old Danielle Fisher became the youngest person to have climbed all of the "seven summits," or the highest mountain on each of Earth's continents.

Her most recent feat--ascending Mount Everest, the world's highest peak--was one tall task: "The higher you go, the harder you have to breathe," says Tom Hornbein, a physiologist at the University of Washington who has also climbed Mount Everest. That's because as you hike to altitudes high above sea level, the air becomes less dense, and contains fewer molecules of oxygen. To help her climb quickly and safely, Danielle breathed bottled oxygen.

Look at the graph (below). How much higher is Mount Everest compared with Africa's tallest summit? On which mountaintop would breathing be most difficult?