Piaget devised

In one of the problems Piaget devised, a child is shown two clay balls of equal size and asked if they both contain the same amount of clay. Next, the researcher rolls one ball into a sausage shape and asks the child if the two shapes still contain the same amount of clay. A preoperational child will say that one now contains more clay than the other and will base his answer on their appearance: "The sausage has more because it's longer now." A concrete operational thinker will say that the two still contain the same amount of material because no clay was added or taken away from either.