You will need:
The shells of 6 eggs
1 tsp very hot water from the tap
1 tsp flour
Wash the eggshells well, so they don’t have any egg left in them. Dry them and grind them with a rock on the sidewalk or other concrete surface. Make sure the rock you’re using for grinding is clean so you don’t get dirt ground in with the eggshells. Grind the eggshells into a fine powder. You’ll need one soupspoonful of this powder to make a stick of chalk.
When you have enough powder to make a stick of chalk, sift or pick out any little bits of eggshell that are still not ground up and throw them away. Scoop the powder into a cup or paper towel and bring it into the house for the next part.
Stir the flour and hot water together in a small dish to make a paste. Put the soupspoonful of eggshell powder into the paste and mix well. It may help to mash it with the back of the spoon. Add a few drops of food coloring if you want colored chalk.
Shape this mixture into a chalk stick. Then roll it up in a strip of paper towel and set aside to dry. (Drying takes about three days.) Then just peel the paper off one end and you’re ready for some sidewalk art.
For really big sticks of chalk, try making 3 times this recipe, and stuff the mixture into an old toilet paper tube. When it’s dry, you can peel away the cardboard as you use it.