> question: What causes tides?

  That's a good question.  The answer isn't immediately obvious,
so I doubt I'll be able to describe it to you in a simple, short
E-mail message.  I'll give it a try, but you should look elsewhere
for more details.

  Start with this: there's a force called "gravity," which causes
objects to attract each other.  The size of the force is large if
two objects are close together, and grows weaker the farther apart
they move.

  Consider the Earth and the Moon: the Earth pulls on the Moon, and
the Moon pulls on the Earth.  In fact, these pulls are what keep
the Earth and Moon moving around each other, instead of flying
off into space on their own.

  Now, imagine that you and two friends -- Alice and Bill -- all of whom
have the same mass, split up and travel to three spots on Earth:

          -  you move to stand on the side of Earth facing the Moon
          -  Alice moves to the side of Earth facing away from the Moon
          -  Bill stands halfway in between, neither facing the Moon
                        nor facing away

  All three of you are pulled by the Moon's gravity, but you feel
_slightly_ different forces.

          -  you are closest to the Moon, so you are pulled harder
                    than Bill or Alice

          -  Bill is farther from the Moon than you, but closer than Alice;
                    so he is pulled less than you, but more than Alice

          -  Alice is farther from the Moon than you or Bill, so she is
                    pulled less than either of you

  Now, all three of you are also pulled downwards towards the Earth's 
surface by the Earth's gravity.  The Earth's gravitational force on you
is much, much stronger than the Moon's, so you remain on the Earth.
But, if you look _very_ closely, you'll see a pattern to the overall
gravitational force that looks like this:

 
                     ------>
                  Bill
               ----------
              /          \
             /            \
      Alice |     Earth    | you                                 Moon
    --->     \            /    --------->
  weak        \          /       strong
               ----------


  The Moon pulls most strongly on you, less on Bill, and least on Alice.

  What does this have to do with tides?  The Earth is covered with water,
of course.  We usually think that the level of the water in the oceans
is the same everywhere .... and it is, to a rough approximation.
But if you look closely, you will see small variations in the height
of the water above the floor of the ocean.  Those variations are
are due to the extra gravitational pull of the Moon.  

  On the side of the Earth facing the Moon, the water is pulled up, away
from the Earth below.  That makes a high tide.  On the side of the Earth
facing away from the Moon, the _Earth_ is pulled away from the _water_
(because the Earth as a whole is closer to the Moon than the water on
that far side); the result is again a high tide, in this case because
the sea floor is being pulled away from the water!  In Bill's position,
neither on the near nor the far side from the Moon, the Earth and water
are pulled equally by the Moon, so there is no extra height to the
ocean.  We call this "low tide".

  For a more detailed explanation, read:

            http://www.badastronomy.com/bad/misc/tides.html
            http://www.astronomynotes.com/gravappl/gravapplb.htm#A9

  Good luck!

                                                  Michael Richmond