According to general relativity, the recipe for building a black hole is dead simple: gather any amount of mass and form it into a ball of a sufficiently small size.11 Of course, even a passing familiarity with black holes leads you to expect that “sufficiently small” means really small, spectacularly small, ludicrously small. And in some cases your expectation is right on the mark. To turn a grapefruit into a black hole, you’d need to squeeze it down to about 10−25 centimeters across; to turn the earth into a black hole you’d need to squeeze it down to about two centimeters across;
