Answer 1:
Good question! Think about it this way:
you've always learned that what goes up, must
come down, right? If you throw a baseball up
from the surface of the Earth, the Earth's
gravity will make it fall back down. Well, it
turns out that's not completely true: if you
throw a baseball away from the Earth fast
enough, it will keep going forever and
completely escape the pull of the Earth's
gravity. The speed at which you need to throw a
baseball to make it leave the surface of the
Earth forever is called the "escape velocity" of
the Earth (it turns out to be around seven miles
per second!).
Now, if you were to make the Earth smaller
but keep its mass the same, the gravitational
force on the baseball at the surface of the
Earth will get stronger. That means that you'd
need to throw the baseball even faster to make
it escape the Earth: we say that the escape
velocity increases as you make the Earth smaller
(but keep its mass the same).
Can we keep going? Can we make the Earth
smaller and smaller, making the escape velocity
bigger and bigger? Nope! Remember that nothing
can go faster than the speed of light, so if we
make the Earth so small that the escape velocity
equals the speed of light (that's 186,000 miles
per second!), then nothing can escape from it.
In fact, it turns out that when you make the
Earth small enough, not even light itself can
escape from it surface; gravity becomes so
strong that if you tried to shoot some light off
of the Earth, it would just fall back down!
Because nothing can get off the Earth, it would
look completely black from far away, and it
would look like a hole that things can fall into
but nothing can get out of, so we call it a
black hole. That's what a black hole is: a
place where gravity is so strong, no even light
can escape.
Hope these help!
