Final answer:
The first stage of Piaget's theory of cognitive development is the sensorimotor stage, which lasts from birth to about 2 years old. Infants learn about the world through their senses and motor behavior, and they develop object permanence.
Step-by-step explanation:
This stage lasts from birth to about 2 years old. During this stage, infants learn about the world through their senses and motor behavior. They explore objects by putting them in their mouths and shaking or banging them to see if they make sounds.
One important milestone in this stage is the development of object permanence, which is the understanding that even if something is out of sight, it still exists. Infants who have developed object permanence will reach for a hidden toy, indicating that they know it is still there.