Child playing peekaboo, © Peter Polak/Fotolia; Child playing vet with Teddy bear, © Gina Sanders/Fotolia.com; Object permanence test, Doug Goodman/Science Source; Blocks silo, Thinkstock; Teenage girl writing on chalkboard, Creatas/Thinkstock; Young Asian boy pouring cooking oil into cake batter, iStockphoto/Thinkstock; Hands open, iStockphoto/Thinkstock; Piaget Conservation-Girl with milk glasses, Bianca Moscatelli/Worth Publishers