And just as Bob, the fish in the water world, is doomed to be swept by the flow of water into the drain hole, so, too, would an astronaut be swept by the flow of space into the singularity. The crucial point is that around the singularity at the center of a black hole is a spherical surface that marks a point of no return. Anything that crosses this surface will never come back out. Nothing inside it, not even a beam of light, can emerge from within it.
