Before the work of Swiss psychologist Jean Piaget, children's thinking was not taken very seriously. Children were often treated as passive recipients, to whom knowledge must be administered like a vaccination. Study kids to figure out how they think and how they learn? Absurd! However, that's just what Piaget did.

We largely owe it to him that today we recognize that children are active participants in their learning. Piaget's work with children led to his identifying four main stages in cognitive development.

Her first two years of life, the sensori-motor stage, a child learns through her body. If she squeezes a toy, and it squeaks, she'll learn that to produce the sound again, she merely needs to squeeze the toy again.

As a child begins to develop language, he moves into the pre-operational stage. Language development leads the child to become more social. Memory and imagination also develop. Ask him if he remembers the time he touched a hot pan, and he may make a sad face.

Around age seven, a child enters the concrete operational stage, in which she begins to develop logic. She begins to understand that changing the shape of water from a tall glass to a wide bowl does not necessarily change the amount of water present.

It's in the fourth and final stage, the formal operational stage, that a child learns to think logically in abstractions. For instance, he can think like a scientist. That is, he can think hypothetically by identifying the various factors involved in a problem, and then test those factors in order to come to a logical conclusion. According to Piaget, not everyone reaches this stage.