1/2 cup salt, 1/2 cup flour, 1/4 cup (give or take) water. Knead until dough forms. Make impression. Bake at 200 for 3 hours. Do every summer and make a stepping stone path :)-- This worked but I would bake it for longer as my "stone" was more like hard rubber..... Also- make sure to have enough paint on hand because it kind of soaks into the grainy-ness of the texture.
These activities would be appropriate for students ages Observe and describe properties of objects. Use age appropriate scientific equipment (e.g., magnifying glasses, thermometer, scales) when participating in scientific experiences.