Children of this age will get an idea in their head and completely ignore other thoughts. The tendency to practice centration is revealed in tasks of conservation. For example, children may conclude there is more water in a shallow dish than in a glass because the dish is wider, even though they have already seen all the water poured from the glass into the dish. Figure 7.2 shows that the child has ignored the greater height of the glass and the demonstration of pouring. As a result of their inability to maintain more than one relationship in their thinking at a time, children make errors of judgment, give inadequate or inconsistent explanations, show a lack of logical sequence in their arguments, and a lack of comprehension of constants.
Classification means that objects can be thought of in terms of categories or classes.
Preoperational children are limited in their ability to classify objects according to categories. The examiner makes certain the children know they are all cats, and that the children can name each breed. Preoperational children also make errors in their thinking because of irreversibility , that is, their inability to recognize that an operation can go both ways. For example, they do not understand that if water is poured from a tall container into a flat container, it can also be poured back again, keeping the same amount of water.