What You Need:
- Clear glass jar or wide mouth bottle (old fashioned milk bottles work great)
- Marble, coin, or checker piece
- 2-inch circle of clear plastic acetate (the kind they use for overhead projectors, available office supply stores)
- 18-inch piece of clear, thin fishing line
- Hole punch
What You Do:
- First, make sure your bottle opening is nice and big—big enough for a marble, checker, or coin to fit through easily.
- Punch a hole through the acetate, and tie the end of the fishing line through it. Then tie the other end around your child’s wrist, and hide it under a long sleeved shirt.
- Have her place the acetate sheet across the opening of the bottle. People up close will be able to see it, but from about 8 or more feet away, it'll be invisible.
- Have your child announce that she can make a marble “float” on top of the bottle, and then fall in—without anyone touching! Have her use her other hand to place the marble onto the acetate. Then have her say a “magic” word, and then quickly move her "string" arm back, pulling the acetate sheet straight back and behind the bottle. The marble will drop straight in…as if by magic!
Your child just demonstrated a core concept in physics, called “inertia.” Once a marble, checker, or coin is placed on top of the plastic, it will be standing still, and because of inertia, it wants to stay that way. When your child yanks the plastic away, the checker, coin, or marble wasn’t ready to keep up. So, plink! Because of another physical principle, gravity, it fell straight into the jar. But your audience doesn’t need to know all that…if all goes well, your kid can keep the secret all to herself. Science is pretty magical, after all.