What causes an object to fall to the earth?

You may not like this answer: Space. As far as we know at this point, gravity is a manifestation of the warping of three-dimensional space created by matter. Earth, like all other massive bodies in the known universe, causes space to bend. It's a strange idea, but if you were to pick one spot in space that was unaffected by mass (a tough task) and followed a straight line to Earth, you would nevertheless bump into Earth, because your line would bend you into it. It's only analogy, but think of space as terrain. It's not always flat. Occasionally there's a dip in the road, sometimes a gully. That's kind of like gravitation; the terrain changes. (If you see a real bright object up ahead and find yourself gaining speed, I'd advise you to rapidly change course.)