Squeeze anything small enough and its own gravity wins forever: a radius appears from which not even light escapes — the event horizon of a black hole. For the Sun that radius is 3 km; for the Earth, 9 millimetres. Nothing is squeezed that far in nature except collapsing stars. Slide the mass and find the point of no return.
The Schwarzschild radius is rₛ = 2GM/c² — the radius at which the escape velocity reaches the speed of light, so nothing can leave. It is astonishingly small: the Sun’s is 2.95 km (it is really 700,000 km across), the Earth’s just 8.9 mm. Only when a mass is compressed INSIDE its own rₛ does it become a black hole. A fail-loud self-check throws unless rₛ = 2GM/c² gives ~2953 m for the Sun and ~8.9 mm for the Earth.
This is the non-rotating (Schwarzschild) horizon from general relativity; spinning/charged holes (Kerr/Reissner-Nordström) differ, and rₛ marks the horizon, not the singularity within. The 2GM/c² value is exact.