4 Common Science 



objects; but the farther apart things are, the less pull 

 they have on each other. 



The earth is very massive, and we live right on its 

 surface ; so it pulls us strongly toward it. Therefore 

 we say that we weigh something. And since every 

 time we roll off a bed, for instance, or jump off a chair, 

 the earth pulls us swiftly toward it, we say that the 

 earth is down. " Down " simply means toward the 

 thing that is pulling us. If we were on the surface of 

 the moon, the moon would pull us. " Down " would 

 then be under our feet or toward the center of the 

 moon, and the earth would be seen floating up in the 

 sky. For " up " means away from the thing which is 

 pulling us. 



Why water does not flow off the earth. It was 

 because people did not know about gravitation that 

 they laughed at Columbus when he said the earth was 

 round. " Why, if the earth were round," they argued, 

 " the water would all flow off on the other side." They 

 did not know that water flows downhill because the 

 earth is pulling it toward its center by gravitation, and 

 that it does not make the slightest difference on which 

 side of the earth water is, since it is still pulled toward 

 the center. 



Why the world does not fall down. And people used 

 to wonder " what held the earth up." The answer, as 

 you can see, is easy. There simply is no up or down 

 in space. The earth cannot fall down, because there is 

 no down to fall to. " Down " merely means toward 

 the earth, and the earth cannot very well fall toward 

 itself, can it ? The sun is pulling on it, though ; so the 



