In this third experiment we also studied the acquisition of the concept of conservation of quantity. A series of exercises was pre sented which aimed at inciting the child to make direct use of his understanding of elementary numerical conservation in order to deal with problems of continuous quantity. A child who is beginning to understand that alteration of the configuration of a collection of ob jects does not change their number, but who has not yet grasped the idea that a change in the shape of, say, a ball of modeling clay does not alter the amount of modeling clay, might be helped to under stand this latter conservation principle if bits of modeling clay were first simply juxtaposed, and then joined before finally being modified in shape. It was hoped that this would make the child realize that the total object is made up of smaller pieces which, even though they may be put together to make different shapes, can be formed again by breaking these up. If number is already conserved, it should not be difficult for the child to go on from this to the idea that the quan tity of substance in the total object remains the same.