A "simple" definition of a black hole would be "A body with escape velocity greater then the speed of light." OK, now let me try to explain this with example.

Theoretically anything with mass can be turned into a black hole. Its all depends upon the Escape Velocity(EV). EV is the speed needed if you want to "escape" from the gravity of a body. For example, escape velocity of earth is 11.2 km/s. If you shoot something(a rocket, cannonball etc) in the sky with less then this speed, it will fall back. But if you shoot it on higher speed then EV, it will be able to go out of the gravitational bounds of the earth.

Now if we keep "squeezing" the earth(told you, theoretically its possible :)) Mass will stay the same but EV will keep increasing. Eventually when you squeeze it down till somewhere about ~0.35 inches(Derived from formula to calculate escape velocity), the escape velocity of earth would be 3,00,000 km/s, i.e. the speed of light. Now according to Einstein's General theory of relativity, nothing can travel faster then light. Thus our squeezed earth is now a black hole. Nothing can escape the gravity of it. Not even light. Our Sun can achieve the same EV if its radius is squeezed to ~2.5 km.

Black holes have immense gravity. Once you are close enough of it, there is no way back. The Universe is full of unbelievable stuffs, the more we learn, the more we get blown off.

Theoretically anything with mass can be turned into a black hole. Its all depends upon the Escape Velocity(EV). EV is the speed needed if you want to "escape" from the gravity of a body. For example, escape velocity of earth is 11.2 km/s. If you shoot something(a rocket, cannonball etc) in the sky with less then this speed, it will fall back. But if you shoot it on higher speed then EV, it will be able to go out of the gravitational bounds of the earth.

Escape velocity(source: wikipedia) |

Black holes have immense gravity. Once you are close enough of it, there is no way back. The Universe is full of unbelievable stuffs, the more we learn, the more we get blown off.