The following news update might be a fun fact to incorporate into one of the Explore: Comets activities (http://www.lpi.usra.edu/education/explore/comets/).
From "Alien Comets Swarm Around Other Stars" by astronomer Phil Plait
We know the Sun is surrounded by literally trillions of small icy comets, most so far away they are invisible even to our biggest telescopes, only easily spotted when they approach the Sun. ...[N]ew observations of six stars show they too have comets orbiting them, which indicates stars with comets may be as common as stars with planets…and those number in the hundreds of billions!
Comets are sometimes called “dirty snowballs”, because they’re composed of rock mixed with ice. That ice itself is a mix of frozen water, carbon dioxide, ammonia, and other things we normally think of as liquids or gases on Earth. But in the depths of space they’re frozen solid. As a comet approaches the Sun, the ice sublimates, turning directly into gas. The gas expands, reflects sunlight, and the comet gets brighter.
Still, stars are very, very far away, and there’s no hope of seeing a comet directly. But if the comet passes directly between us and the star, some of the starlight gets absorbed by the comet's gas. Different chemicals absorb different colors of light, so by carefully analyzing the starlight, the presence of a comet can be detected.