I worked on videotaped developmental psych studies in the 80s; one of them was around kids grasping the idea of transformation based on Piaget's theories. Pouring water from a wide beaker into a tall one would get nt kids up to a certain age to say there was now more water. Same thing with clay, object permanence, counting, etc. I had as hard a time then understanding how a kid could think the amount of water changed, as they did understanding that it didn't change. Some would argue against the value of a child going to school before they entered Piaget's "Conrete Operations" phase.