Suppose you want to buy a cart horse. Of course for most of us, this isn't likely but for a lot of people it is much like buying a used car. You need to look it over to see that there are no dings and scratches and then you need to see how well it accelerates and so on. But how to do that? At some of the horse fairs they tie a wheel of a cart to keep in from rolling and then ask the horse to move it quickly.