It seems like common sense that the largest animals should move the fastest. They have the largest muscles, after all. But that's not the case. Cheetahs are far from the biggest animals on the planet, but they top the charts at around 70 mph. Larger animals, it turns out, simply can't reach the zippy speeds their muscles should theoretically be able to propel them to—they tire out long before that. An elephant's enormous legs have far more fast-twitch muscles (the kind that help you sprint), but the massive beasts run out of oxygen before they can use those fibers optimally.