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Exploding Sidewalk Chalk ~ A little art and a little science. Mix vinegar, liquid watercolor paint (or food coloring) and corn starch together in a ziplock bag (keep it liquid, no amts specified). Pour a small amt of baking soda onto a paper towel square and fold it up. When ready, add baking soda bomb into bag, seal immediately, shake and plop on ground.
"Exploding Sidewalk Chalk - FUN!"--this looks totally awesome :) definitely great for outdoor play, and I imagine you could just hose down the pavement to wash it clean (though you might want to test that). I cannot wait to give this a shot :)