"If you could experience being dead for one day to learn what happens in the afterlife, and were guaranteed to return to life the following day, would you do it? Why or why not?"
A black hole is often defined as an object whose "escape velocity" exceeds the speed of light.
The "escape velocity" of an object depends on how compact it is; that is, the ratio of its mass to radius. A black hole forms when an object is so compact that, within a certain distance of it, even the speed of light is not fast enough to escape.