Whenever anything moves, it pushes something else in an opposite direction. When you row a boat you can notice this; you see the oars pushing the water backward to push the boat forward. Also, when you shoot a bullet forward you can feel the gun kick backward; or when you pull down hard enough on a bar, your body rises up and you chin yourself. But the law is just as true for things which are not noticeable. When you walk, your feet push back against the earth; and if the earth were not so enormous and you so small, and if no one else were pushing in the opposite direction, you would see the earth spin back a little for each step you took forward, just as the big ball that a performing bear stands on turns backward as the bear tries to walk forward. [Illustration: FIG. 38. The horse goes forward by pushing backward on the earth with his feet.] The usual way of saying this is, "Action and reaction are equal and opposite." If you climb a rope, the upward movement of your body is the action; but you have to pull down on the rope to lift your body up. This is the reaction. Without this law of action and reaction no fish could swim, no steamboat could push its way across the water, no bird could fly, no train or machine of any kind could move forward or backward, no man or animal could walk or crawl. The whole world of living things would be utterly paralyzed. [Illustration: FIG. 39. As he starts to toss the ball up, will he weigh more or less?] When _anything_ starts to move, it does so by pushing on something else. When your arms start to move up, they do so by pushing your body down a little. When you swim, you push the water back and down with your arms and legs, and this pushes your body forward and up. When a bird flies up into the air, it pushes its body up by beating the air down with its wings. When an airplane whirs along, its propeller fans the air backward all the time. Street-car tracks are kept shiny by the wheels, which slip a little as they tend to shove the track backward in making the car move forward. Automobile tires wear out in much the same way,--they slip and are worn by friction as they move the earth back in pushing the automobile forward. In fact, if there are loose pebbles or mud on the road, you can see the pebbles or mud fly back, as the wheels of the automobile begin to turn rapidly and give their backward push to the earth beneath. [Illustration: FIG. 40. Action and reaction are equal; when he pushes forward on the ropes, he pushes backward with equal force on the seat.] Here are a couple of experiments that will show you action and reaction more clearly: