Crayon hot rocks: If you have not done this yet, you need to. Simply warm up the rocks in the oven (I did 220 for 10 minutes), then put them on a stack of newspapers (or something else you don't care if it gets crayon in it) and let the little ones go to town. Just remind them it is hot. I have done it with 2 1/2 year olds with no problem. My little lady often brings in rocks for me to "cook" so she can paint them
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The 25 Coolest Science Experiments for kids!