According the children will apply the theory in their development at preoperational stage. The preoperational means the children in this stage cannot thinking logically yet and there are four cognitive deficits in this stage which are animism, hierarchical classification, egocentrism and conservation. Animism means the objects are lifelike qualities such as the children will think the cars are clever because the star or sun is following them. Hierarchical classification happens when children classify the things into categories. For example, a kitty is a cat and not all cats are kitty. The children in this stage have trouble to classify objects in a hierarchy, for example, they think all kitties are cats and her mother is not a woman but her mommy. Next, the egocentrism is the children try to give their point of view and think other people also think like that. For example the three mountain task when a child is given a three-dimensional model of three mountains that have different objects on it. Then the child will give different pictures and pick the pictures that the doll sees but the children will choose the picture that he sees not the doll sees. The children in this stage will show perspective skills on tasks but not others. Conservation is the objects have fixed mass, number and volume. For example, when an apple is cut into two, the children will say the apple is big and more.Next is the concrete operational stage. In this stage, the children apply this theory more than other stages. explains the children become well in logical reasoning like the child understands the volume object is always fix and understand the relationships between families according to family tree. They also can understand the world more objectively, for example if they are collecting the stamps, they will divide the stamps into certain categories and themes. Besides, they learn how to use the full address and sent letter to everyone they want. In this stage, they can complete the bottle and mountain tasks. For example in bottle task, a picture of a bottle and liquid at the bottom and three others bottles are presented to the child. The result is, the child can draw the water would be in the next three bottles. In mountain tasks, the child is able to draw people, house and mountain. Then, for formal operational stage, the child will think more logic and abstract ways. They will think about their future and solve their own problems systematic. The formal operation thought is important because the children will understand science and mathematics subjects for example in controlled experiment, they should know how to think logic and in algebra, the variable x is used for many numbers.