How Does A Canyon Become Grand?

How do you make a Grand Canyon? I was in Arizona recently for Phoenix Comic-Con, and had the amazing pleasure of seeing one of Earth's greatest natural wonders… the Grand Canyon. More than a mile deep, and several miles across, it just defies belief. But I couldn't help but think, the Colorado River down at the bottom isn't that big. How did it cut a canyon so massive? How old is the Grand Canyon?