The choice of gait, that is whether we walk or run, comes to us so naturally that we hardly ever think about it. We walk at slow speeds and run at high speeds. If we get on a treadmill and slowly crank up the speed, we will start out with walking, but at some point we will switch to running; involuntarily and simply because it feels right. We are so accustomed to this, that we find it rather amusing to see someone walking at high speeds, for example, during the racewalk at the Olympics. This automatic choice of gait happens in almost all animals, though sometimes with different gaits. Horses, for example, tend to walk at slow speeds, trot at intermediate speeds, and gallop at high speeds. What is it that makes walking better suited for low speeds and running better for high speeds? How do we know that we have to switch, and why don’t we skip or gallop like horses? What exactly is it that constitutes walking, running, trotting, galloping, and all the other gaits that can be found in nature?