I always thought the different 'phases' of the moon was caused by the earth's shadow on it. This made sense when looking at the 1st quarter, but as the moon 'gets bigger' the visible part is no longer crecent shaped. Where am I going wrong in my thinking?
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Here are a few sites that explain it well:
http://www.hometrainingtools.com/art...ching-tip.html
http://en.wikipedia.org/wiki/Moon_phases

I remember a do-it-yourself, at-home demonstration that is excellent at showing why the moon has phases. You need a round ball, a flashlight, and a dark room.

O Set the flashlight on a shelf, at approximately eye level, and turn it on. This is the sun.
O Stand in the beam of the flashlight. Your head is the earth.
O Hold the ball out at arm's length. That's the moon.

Turn in circles on the spot, keeping the ball in front of your face at arm's length. The moon moves around the earth in its orbit. Observe how the shape of the lighted portion of the moon changes, depending on the angle between the earth, the moon, and the sun.

If you get the angles right, you can even show why solar and lunar eclipses happen, in a demonstration simple enough for a child to understand.