If you take a round shape and cast a shadow across it, you are demonstrating what happens during an eclipse.
If you take a ball, paint half of it black, and turn it slowly around an axis so different amounts of the black part show, you are demonstrating what happens during the moon's phases. Or even better yet... you stand in one spot, have the white part always face the same wall, move the ball around you.
Very important not to confuse the two. Both demonstrations will show a crescent kind of shape at one point, but that's the last time they'll have anything in common so you'll have a heck of a time explaining the gibbous phases if you mess this up. And apparently it's a common misconception that the moon's phases are caused by the earth's shadow. Think of the children.
And because I haven't talked about it yet this year, also think of the children when you teach them about snowflakes, make sure your snowflakes have six points. Don't screw up the future.