These are known as pony beads in the arts and crafts industry. These beads have been treated with a special, ultraviolet light-sensitive pigment. When you take them out into the sun or put them under a UV lamp they change from an opaque white to one of five brilliant colors. You can use these to introduce colors, talk about energy conversion, and experiment with polarizing lenses as well as different kinds of sun block.
Demos and Labs: Painting With Sunshine