Final answer:
The brother is likely in the preoperational stage of cognitive development, where children do not understand conservation and exhibit egocentrism, making them believe the quantity of clay has changed with its shape.
Step-by-step explanation:
Your little brother is likely in Piaget's preoperational stage of cognitive development, which spans from approximately 2 to 7 years old. In this stage, children begin to use symbols to represent objects and engage in pretend play. However, they do not yet understand the concept of conservation, which is the understanding that properties such as mass, volume, and number remain the same despite changes in the forms of objects. Your brother's reaction to the ball of clay being broken into smaller pieces indicates his belief that the quantity of clay has diminished, demonstrating a lack of understanding of conservation.
During the preoperational stage, children also display egocentrism. They are unable to take on the perspective of others, believing that others see, think, and feel exactly as they do.