You asked:
If the moon causes the tides, then why are there two high tides and two low tides every day? — Leigh, Raleigh, NC

Marshall Brain Answered:
A tide table like this one shows you exactly when the high tides an low tides will arrive every day, and it is true that, most days, there are two high tides and two low tides. There aren’t exactly two every day because the moon takes about 28 days to orbit the earth once, and most months have two or three more days than that. (The exact interval is 12 hours 25 minutes from one high tide to the next).

The moon is the source of the tides. It’s gravitational pull affects the level of the water in the oceans. In this case, a video paints a thousand words, as this video shows you exactly what is happening at high and low tides:

The Flash animation on this page os also very helpful.

For more info see What causes high tide and low tide? Why are there two tides each day?