_But_, if there is no friction and you want to stop, you cannot. Suppose you are in an automobile when all friction stops. You speed along helplessly in the direction you are going. You cannot steer the machine--your hands would slip right around on the steering wheel, and even if you turn it by grasping the spoke, your machine still skids straight forward. If you start to go up a hill, you slow down, stop, and then before you can get out of the machine you start backward down the hill again and keep on going backward until you smash into something. A person on foot does not fare much better. If he is walking at the time friction ceases, the ground is suddenly so slippery that he falls down and slides along on his back or stomach in the same direction he was walking, until he bumps into something big or starts to slip up a slope. If he reaches a slope, he, like the automobile, stops an instant a little way up, then starts sliding helplessly backward. Another man is standing still when the friction is turned off. He cannot get anywhere. As soon as he starts to walk forward, his feet slip out from under him and he falls on his face. He lies in the same spot no matter how he wriggles and squirms. If he tries to push with his hands, they slip over the rough ground more easily than they now slip through air. He cannot push sideways enough even to turn over. If there happens to be a rope within reach and one end is tied to a tree, he might try to take hold of the rope to pull himself along. But no matter how tightly he squeezes, the rope slips right through his hands when he starts to pull. If, however, there is a loop in the rope, he can slip his hand through the loop and try to pull. But the knots with which the rope is tied immediately come untied and he is as helpless as ever. Even if he takes hold of a board fence he is no more successful. The nails in the board slip out of their holes and he is left with a perfectly slippery and useless board on the ground beside him for a companion. As it grows cold toward evening he may take some matches out of his pocket and try to start a fire. Aside from the difficulty of his being unable to hold them except by the most careful balancing or by shutting them up within his slippery hands, he is entirely incapable of lighting them; they slip over the cement beneath him or over the sole of his shoe without the least rubbing.