It is centrifugal force that keeps the water in the pail. It depends entirely on inertia. You see, while the pail is swinging upward rapidly, the water is moving up and tends by its inertia to keep right on moving in the same upward direction. Before you get it over your head, the tendency of the water to keep on going up is so strong that it pulls on your arm and hand and presses against the bottom of the pail above it. Its tendency to go on up is stronger than the downward pull of gravity. As you swing the pail on backward, the water of course has to move backward, too; so now it tends to keep on moving backward; and when the pail is starting down behind you, the water is tending to fly out in the backward direction in which it has just been going. Therefore it still pushes against the bottom of the pail and pulls away from your shoulder, which is in the center of the circle about which the pail is moving. By the time you have swung the pail on down, the water in it tends to keep going down, and it is still pulling away from your shoulder and pressing against the bottom of the pail. [Illustration: FIG. 36. Why doesn't the water spill out?] In this way, during every instant the water tends to keep going in the direction in which it was going just the instant before. The result is that the water keeps pulling away from your shoulder as long as you keep swinging it around. _All whirling things tend to fly away from the center about which they are turning._ This is the law of centrifugal force. The earth, for example, as it swings around the sun, tends to fly away from the center of its orbit. This tendency of the earth--its centrifugal force--keeps it from being drawn into the sun by the powerful pull of the sun's gravitation. At the same time it is this gravitation of the sun that keeps the earth from flying off into space, where we should all be frozen to icicles and lost in everlasting night. For if the sun's pull stopped, the earth would fly off as does a stone whirled from the end of a string, when you let go of the string. The moon, in like manner, would fly away from the earth and sun if _gravitation_ stopped pulling it, but it would crash into us if its _centrifugal force_ did not keep it at a safe distance. Have you ever sat on a spinning platform, sometimes called "the social whirl," in an amusement park, and tried to stay on as it spun faster and faster? It is centrifugal force that makes you slide away from the center and off at the edge. [Illustration: FIG. 37. An automobile race. Notice how the track is banked to keep the cars from overturning on the curves.]