Having established relativity as the replacement for precise mechanics, Einstein delved into the irregularities between calculations and observations in the realm of Newtonian gravity. Newtonian gravity describes the gravitational interaction between two bodies as being a function of their respective masses and the distance between them. The problem with Newtonian gravity arises when the mass of one of the objects changes suddenly. According to the Newtonian model, if the sun were to explode, the gravitational effects would be felt on Earth instantly. In such a situation, no time passes before changes are rendered. This requires some force that moves faster than light, which seems to be impossible. Therefore, Einstein surmised, there must be another way to look at gravity.
Newton was aware of this problem, and it troubled him. Newton envisioned some sort of tether between the bodies that allowed gravitational interaction. Einstein broke from this theory by instead imagining space as being an interactive sheet. A common metaphor is that of a sheet of rubber stretched over a bowl. When nothing is present, the sheet remains flat. Should a mass be placed on the sheet, the sheet reacts by sagging accordingly. If a second object is added, it will naturally move toward the first object, both reacting with the sag caused by the first object and creating a sag of its own. Click the picture to the right to see this in action with an onion and a ping pong ball.
The metaphor is useful for visual purposes, but is inherently flawed. First, the sag in the example is created by the gravitational pull of the Earth, a third body that interacts with the onion and ping pong ball. In reality, no such third party exists. Furthermore, the example exists with a two dimensional sheet, whereas Einstein's model occurs in all four dimensions.
Black holes are a possible effect of the theory, as they warp the fabric of spacetime so severely that nothing can climb out. Their existence, however, is still the subject of debate.