So what is a black hole in this context? Clearly the more matter I put in the center of the sheet, the deeper the well that I create, and the harder it is for matter to "climb out" . According to Einstein's theory, if I pack enough matter into a small enough volume, the well will get so deep that the matter inside can never escape. A circle of no return forms. Any matter that passes the point of no return can no longer escape to the outside world. It necessarily keeps collapsing, moving towards the center. The well gets deeper and deeper until finally a hole is literally torn in the fabric of spacetime: the density of matter at the center becomes essentially inifinite, at least to the extent that Einstein's theory of gravity is still valid. Thus, what I mean by " a hole in the fabric of spacetime" is: a tiny region of space where the known laws of physics break down. A black hole is a region of space so tightly packed with matter, that nothing, not even light can escape. Hidden at its (crunchy?) center is a tear in the fabric of spacetime. Anything that falls into this region of space is irrevocably lost to the rest of the universe. No light can emerge or pass through this region, so it appears totally black. In some sense therefore, a black hole marks a boundary to spacetime: a horizon beyond which no one can see without travelling through it. This radius of no return is called the event horizon of the black hole.