Refraction: Spot the Penny

Place a penny in the bottom of an opaque cup. Set the cup on a table or counter. Keep your eye on the penny as you step back from the cup. Continue to step back until you can no longer see the penny. Pour water into the cup (if you can reach, or have someone else pour it for you), and as it fills the penny will appear due to the refractive nature of water.

If you have difficulting with the penny moving while you're pouring the water in, you could glue the penny to the bottom of the cup ahead of time. You'll have it set up for years to come, and you'll know for sure that you're seeing the penny because of refraction, not because it moved.