Originally Posted by Matt L

First, lest we forget, gravity is *only a theory*. While it is true that the forcasts of this theory see to be entirely reflected in the way the world works, this fact alone changes nothing.

Second, gravitational theory is not understood, by anyone, much less by your sister when she was seven. Einstein tried to equate it to the other three forces (so-called Field Unification), and failed. While we have been able accurately compute the effects for a very long time, we have no idea why these effects occur.