hmmm...just a guess but could it be when the moon goes down and then when the moon goes up?

Nope! The moon circles the earth once a month (lunar!) The earth rotates on it's axis once per day!

Basically. Tides are caused by the pull of the MOON and the SUN and because one is always opposed to the other ,thus night and day,we get two tides.This is a simplified explanation. The earths rotation also has an effect.

Tides are caused by the gravitational interaction between the Earth and the Moon. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Since the earth is rotating while this is happening, two tides occur each day.

I once got into a discussion on this topic with some colleagues of mine when I was teaching at a college in Utah. In order to understand why we have two tidal bulges, it is necessary to first understand calculus and vector mathematics. At the time my knowledge of these two subjects was such that I could only barely follow the explanation. I would not have wanted to try to recreate it. When we got done with the discussion I ask the instructor leading it how we were supposed to explain it to our introductory astronomy students. He looked at the board, then he looked at me and said, "You don't." The up shot of this is that the explanation is very complicated. So, suffice it to say that the gravitational force of one body acting on another decreases with the distance between them. The Earth is big enough that the front side is attracted more strongly than the back side. This causes a stretching effect along the line between the Earth and moon. This stretching effect is what causes the two tidal bulges.

Think of it like and egg. The yolk is earth and the strong pointy side is water being pulled toward the moon. The other side also has a bulge and that is the other high tide.

Which of you came up with that answer, McCain or Palin? We are in deep Doo Doo aren't we.

The Moon causes a bulge in the sea on both sides of the Earth, and as the Moon appears to circle us once per day due to the spinning Earth, you get two tides a day offset by about an hour a day, which is the additional motion of the Moon in one day, compared to the Earth. But why is there a bulge on the far side of the Earth to the Moon? Suppose we change Earth's spin rate so that it matches the orbit of the Moon so the Moon is directly overhead all the time. There is still a bulge in the direction of the Moon and one in the opposite direction. The Moon's orbit hasn't changed, it doesn't come crashing down on our head. Why not? From our point of view, the Moon is being pulled towards us by gravity, but it is being flung outward by centrifugal force as the Earth and Moon rotate. The centrifugal force is an artifact caused when you measure force while you are rotating. The two forces balance to keep the Moon from crashing on our head. But the centrifugal force is proportional to the distance, while the force of gravity is inversely proportional to the square of the distance. They only cancel perfectly at the center of rotation. This means that things nearer the moon will feel a small upward force and things further from the moon wil l feel a small outward force compared to the Earth. This is also true in the non-rotating system. This force ... is called the tide.