A black hole is nothing more than the extreme limit of gravity. It's
a very large collection of matter taking up a very small space.
Think of it this way. Einstein showed us that all matter curves space,
right? Well, because it's so hard to visualize this curvature (because
space-time occupies the accursed 4th dimension with width, height, length,
and time) scientists have come up with as easy way to think about it.
Imagine space as being on 2 dimensional... like a piece of paper or a sheet
of plastic. Then, with nothing in it, the space would be flat, like this:
Look at the images below of objects in our Universe positioned in
space-time. Choose the image that nest represents the result of a) our
Earth, b) the Sun, c) a white dwarf, d) a neutron star, and e) a black hole
as it would appear making a "dent" in space-time.
This activity was developed by Jacqueline Slay, Largo High School, Largo, MD