Brains are powerful and complex! They don't come fully equipped from the get-go. The brain goes through many changes in the first couple decades of a human's life that influence how people act and learn.
Humans go through natural stages of development where they express different patterns of behavior. Though people grow at different rates, they all follow the same basic sequence.
Up until about 6 months of age, infants lose awareness of an object as soon as they are unable to perceive it. The child eventually understands that the object exists and can even be manipulated while away.
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Different concepts (such as language) are easier to pick up at certain points in the brain's development. This is due to the brain's plasticity during earlier stages.
You kids need to slow down!
Humans make "shortcuts" when categorizing things. When encountering new information, the brain first tries to make the information fit into an existing category.
Since children are less developed and experienced, they have to make a lot of new shortcuts to make sense of new information. People can also develop more complex understandings of old information.