Egg cartons are great for sorting. Today O got her egg carton off her shelf and put shells in it. She was very engrossed in this work. I prompted her to take the shells out and put them back in the basket after she was finished. She counted as she put them back...2, 3, 4, 6, 9. For some reason she refuses to say 1 :) See Montessori Mama's blog for more fun things to do with egg cartons.