Can you tell me what causes eclipses of the sun and moon? Why do they not happen every month?
A lunar eclipse occurs when the Moon enters the Earth's shadow. A solar eclipse occurs when the Moon's shadow falls on the Earth. They do not happen every month because the Earth's orbit around the sun is not in the same plane as the Moon's orbit around the Earth.
If you were to draw a little Earth in orbit around a little sun on a piece of paper, then you would not be able to accurately draw the Moon's orbit on that same piece of paper. Sometimes the Moon will be above the paper, other times below it. Only when the Moon is crossing the plane of the Earth's orbit (the paper) just as it is lining up with the Earth and Sun will an eclipse occur.
More details on this phenomenon can be found at this page from the Earth View website.