Phases of the Moon
Swear not by the inconstant moon.
The moon changes it's shape in the sky, but the moon itself stays the same. How can this be?
Materials
- black table tennis (Ping Pong) balls
- white table tennis ball
- push pin
- unfolded large paper clip
- table lamp without its shade
- bright bulb
- very dark room
Construction
- Use the push pin to poke a small hole in black table tennis ball.
- Unfold the paper clip but leave a bit folded to make a handle. Poke the end of the clip into the ball and push it all the way through to the other side. Try to go directly across.
- Set up the lamp in the room. The activity works better if the bulb is a little higher than the head of the people doing the activity.
See and Notice
Viewing as someone on the earth
In your darkened room, put the black table tennis ball in hand, and stretch your arm so that it is about a foot from your face and a little higher than your eyes.
Spin slowly around in a circle (counterclockwise from above), looking carefully with how the light appears on the ball. You observing as if you are a person on the earth! You will notice, that the ball looks totally black when it is between you and the sun (lamp) 
and as you rotate away from the sun, you'll see a slight, bright crescent. 
You might notice that the crescent looks a little like an arrow, and it is pointing at the sun.
As you continue to turn, you'll notice that the bright crescent gets larger and becomes less crescent shaped. Soon it looks like a half circle but with shaded fuzzy line between the bright and unilluminated sides of the sphere. The line called the terminator looks like it is moving from right to left on the ball. After you've turned about 2/3 of the way, you'll see that the terminator is no longer straight but curved again with an even fuzzier edge. Finally, when you facing directly away from the sun, the ball will be completely illuminated and will look like a bright circle. 
Wait you say that yours doesn't look like that? Your ball is in the shadow of your head? Lift the ball a little higher, and it will be in the light again. Why does your head create an eclipse when it is rare in the real world? Your head is much larger than the table tennis ball than the earth and the moon, and the ball is much closer to your head than the moon is to the earth. So in the real world, the earth's shadow is way smaller in proportion to the moon than your head's is to the table tennis ball. Equally important the moon's orbit around the other is tilted a little bit compared to the earth, so the moon is rarely directly behind the earth. It is usually a bit higher or lower than the earth's shadow.
Keep on turning and you'll see the terminator start on the right side. As you continue to turn, you'll see the terminator move to the left making the moon look smaller and smaller until you once again the ball is directly between you and the lamp.
Viewing as someone looking down on the earth and moon
Turn the ball upside down, and move the ball in a circle near the light. Notice how the ball is always illuminated on the one side facing the sun.