Physics Competition

I'm preparing a safe that is comprised of physics-based puzzles for a competition. Having a little bit of trouble figuring out what would make a good physics principle to base a puzzle off of. Ideally, it should be able to be solved relatively easily if you are familiar with the principle but be very difficult if you don't. One example might be using total internal reflection to shine a light at a receptor through an optic cable or use magnetism and a faraday train to send a projectile down a tube. Any ideas for a puzzle?