Seed-Balls (a.k.a. Seed-Bombs) are a seed distribution technique. Evidently, there are guerrilla gardeners out there, who make these and lob them over fences into neighbors' neglected yards, along the highway, or into vacant lots. The seeds are protected, from those who would eat them, by the clay, and sprout on their own once enough water has fallen to release the clay. I'm making them to give out as favors at my middle child's birthday party, and I could definitely see this being a great idea to do with kids for Earth day or a garden party.
This recipe is from http://heavypetal.ca/archives/2007/03/how-to-make-seedballs/, but there are lots of variations out there.
Mix 5 parts dry red clay, 3 parts dry organic compost, 1 part seed, and 1-2 parts water (just enough water to form a mix that will hold together). Pinch off small bits of the mixture and roll into 1/2 inch balls. Let them sit for a few days until they're completely dry. The Heavy Petal gardener used a 16 oz measure and said the recipe made enough for 300 (!) seed balls.
The red clay you can get from a well-stocked craft store or from a pottery supply store--remember, you want the powder, not the pre-mixed stuff. The seeds can be anything you want, although it's a good idea to include only native plants, or at least non-invasive plants, if you're planning to use these to re-plant the world guerrilla-style.
Here's a little more explanation about seed balls: http://heavypetal.ca/archives/2007/03/a-brief-history-of-the-seed-ball/