The difference between primary and secondary circular reactions is that secondary circular reactions involve the infant responding to people and

The answer is "secondary circular reactions involve the infant responding to people and objects in outer world."

An example is that when a mother hands the infant a duck or something he squeezes it, sometime creating a sound which the infant feels like responding to and that is why he/she will repeat the action again and again.
The idea was presented by a Swiss scientist Jean Piaget and is known as Piaget's stages of sensorimotor intelligence.

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