Anyone try this? I'm tempted. My boys love drawing on the concrete walks and I save my eggshells. Normally I pulverize them in an old coffee grinder and compost or feed them back to the hens but this sounds like fun.
http://www.make-stuff.com/kids/make_your_own/eggshell_chalk.html
http://www.make-stuff.com/kids/make_your_own/eggshell_chalk.html