Presenter notes: Drop a duster and let it hit the floor. Ask why it fell. Somebody says gravity, and everybody nods, and that is the moment to stop them. Ask what gravity actually is, and the nodding stops. Then ask a harder one. If gravity pulls things down, what does down mean for somebody standing on the other side of the world? Their down and our down point in opposite directions. Let that sit for a moment and then promise them the answer, along with the reason your mass would survive a trip to the Moon and your weight would not.