At what stage did Piaget believe that children fully understand how to consistently classify objects and understand the relationships among classes?

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Piaget believed that children fully understand how to consistently classify objects and understand the relationships among classes during the concrete operational period, which occurs between the ages of 7 and 11. During this stage, children become capable of logical thinking and can perform mental operations on concrete objects or events. They can also understand classification, seriation, and conservation.

At What Stage Did Piaget Believe That Children Fully Understand