Here's a neat experiment that will fascinate as well as educate. At first, students see only a glass containing water and a floating paper clip. But when they lift the paper clip out of the water, they find it's attached to a crystal. (Ghost Crystals) What's the secret? Carbon-based polymers that absorb up to 300 times their weight in water. Since Phantom Crystals are made up almost entirely of water, light passes through them without being refracted. When submerged, they appear invisible because the crystals have the same index of refraction as water. But when exposed to air, they're clearly visible, because air's index of refraction is very different from that of water.