Mmmm....our third grade students study fossils as as part of their Earth materials unit. This Martha Stewart recipe for fossil cookies looks like it might be something that can be worked into the unit.
I don't like the fact that you have to freeze the cookies first (making the project a two day event) and (unless you have a toaster oven in the classroom) you would have to make them at home.
Some schools don't allow hot plates or toaster ovens. If you aren't sure...DON'T ASK (easier to get forgiveness then permission). I feel TOTALLY fine with bringing in a toaster oven for this one activity and then taking it back home. I would most likely want a parent to help "watch" the toaster oven but the darn things have to bake for 30 minutes.
If any cooks out there have a better idea please let me know.