Sidewalk Chalk

1 cup plaster of paris (do not pack)
almost 1/2 cup cool water
Liquid tempera paint in various colors
(your kid's teacher may "donate" the paint in exchange for a few
sticks of chalk)
Margarine tubs or other disposable mixing containers
Disposable molds
(we used toilet paper rolls with tape covering one end to make a
"cup")

This stuff won't work on chalkboards, it's too hard, but it works great
on sidewalks and lasts MUCH longer than store bought stuff. I made it
for christmas presents this year for kids, and it worked great!
Warning: this is a fun and messy project!

Pour plaster into a container. Using a disposable stick, stir in most
of the water. Add 2 or 3 tablespoons of liquid tempera, mixing well,
especially at the bottom. Add a little more water so the mixture
thickens, stir well, and pour into the molds. Remove the molds after
the chalk is completely dry.

Source: Amy Dacyczn's Tightwad Gazette II