Irreversibility is one of the characteristics of behaviorist Jean Piaget's preoperational stage of his theory of child development. It refers to the inability of the child at this stage to understand that actions, when done, can be undone to return to the original state. Thus, the child cannot use this understanding to solve problems.
Piaget exemplified irreversibility by using the example of two glasses of different shapes and sizes. When water is poured from one glass to the other, the child in the preoperational stage is asked if the volume of water in the second glass is the same as the volume had been in the original glass. Irreversibility is the inability of the child to understand that the volume can be proven to be the same by undoing the action and pouring the water back into the original glass.
According to Piaget, the preoperational stage in child development occurs when the child is between 2 and 7. Along with irreversibility, the other main characteristics of this stage are egocentrism, which signals the inability of a child at this stage to conceive that another person looking at a three-dimensional object from another perspective may have a different conception of the object, and centration, which defines the inability of a child to focus on one detail of a problem without neglecting another.