Because we can feel that things are heavy, we think of weight as being a fixed quality in an object, but it is not really fixed at all. If you could take a one pound packet of butter 4,000 miles out from the earth, it would weigh only a quarter of a pound. Why would things weigh only a quarter as much as they do at the surface of the earth if we took them 4,000 miles out into space? The reason is this: All objects have a natural attraction for all other objects; this is called gravitational attraction, but this power of attraction between two objects gets weaker as they get farther apart. When the butter was at the surface of the earth, it was 4,000 miles from the center (in other words the radius[半径] of the earth is 4,000 miles). When we took the butter 4,000 miles out, it was 8,000 miles from the center, which is twice the distance. If you double the distance between two objects, their gravitational attraction decreases (减少) two times two. If you treble () the distance, it gets nine times weaker (three times three). If you take it four times as far away, it gets sixteen times weaker (four times four ) and so on. The best title for this passage is______.