When we see a fly flying towards the ceiling, or mice running around in a circle, it does not particularly surprise us. On the other hand, if we had seen books suddenly hovering towards the ceiling and clinging to it, or tennis balls starting to dance around each other, we would have been absolutely surprised. This is because as far as we know, living things have the ability to move, but inanimate objects do not have such ability.
Inanimate matter cannot move on its own. Put a stone on the floor, and it will stay forever at that point if it is not moved away. This is quite understandable, and where will inanimate, dead matter, get the ability to move?
But in fact, inanimate objects move all the time. Books do not float towards the ceiling, but they certainly float or fall, towards the ground, like any other object. Tennis balls do not dance around each other, but stars and suns do. The entire universe is actually a system of lumps of matter of varying sizes, from the subatomic particles to the stars, all of which are in constant motion. Because we are so used to it, we do not pay attention to the wonder of it: what drives those inanimate blocks of matter? What makes them move in such precise orbits? How can inanimate matter move?
These questions were known to occupy Isaac Newton, according to legend, following an apple that fell on his head. Newton studied the motion of objects, on and off the earth, and following his research, formulated the equations describing gravity and the three laws of motion, which form the basis of classical mechanics.
Nowadays every child knows to say that objects fall because of gravity, and every person with a basic education knows that these laws are also what is behind the movement of the stars, the tides, and other phenomena. Einstein's theory of relativity later changed some of the Newtonian conceptions, but these are still considered accurate and relevant to most routine realities.