Jean Piaget was a Swiss developmental psychologist who was most famous for his theory of cognitive development. Piaget believed that as children grow biologically, they also develop cognitively, and that this development occurs as an interaction that happens in specific, chronological stages. Piaget's model not only influenced theory of developmental psychology, but also impacted education. Today most pre-school and primary school programs are modeled after Piaget's ideas, so that teachers can present materials that are challenging but appropriate to a child's natural development.

Show the child an object, such as a favorite toy. Place the object behind a screen, within reach of the child. If the child is able to find the toy by reaching behind the screen, he has obtained object permanence. The first stage of cognitive development is the sensorimotor stage, and is completed when the child obtains object permanence, or the ability to understand that an object still exists when it is not seen.

Engage the child in pretend play. Children in the preoperational stage acquire the ability to enact scenarios with plots and roles, such as pretending to be "mom" or "teacher." The preoperational stage occurs when a child is a toddler and begins developing the use of symbols and language. If the child is able to respond to you with speech and engages in imagined pretend play, she is likely in the preoperational stage.

Show the child two glasses of the same size, filled equally with water. Ask him if they contain the same amount. When he confirms this, take one of the glasses and pour all of the water into a taller, thinner glass. Then, ask him, "Do these two glasses contain the same amount of water, or does one have more?" A child who says that the taller glass contains more water has not yet obtained understanding of concrete operations. A child has obtained concrete operations, Piaget's third stage of development, when he begins understanding concrete measures of conservations such as liquid, number, length, mass and weight.

Test an adolescent's ability to deal with mathematical concepts such as fractions, percentages, decimals and ratios. Ask her to engage in a debate about current social and political issues and to take multiple sides. A person who is able to demonstrate proportional reasoning and imagine different logical arguments shows signs of having obtained formal operations. Formal operations involve the type of thinking that is required by late high school and early college courses. Formal operations are marked by the ability to use symbols logically and develop abstract concepts.