 So, we are going to learn developmental theories and under the umbrella of developmental theories we will see cognitive theories. So, at birth the brain of a baby is only 25% of the weight of an adult's brain which is 1.5 kg and by the age of 3 it is 90% of the adult's weight and by the age of 6 almost as large as it will ever be. You are seeing in the picture that when a baby is born, the neurons of the brain, the structure of the brain cells and they are connected to each other by neurons, there are some parts that are born in it, then they shift from one part to another in one cell, in one neuron to another in the other neuron. So, at birth time they are still loose. Then, as it happens in one month, they get complex at the same time and when we are almost 4 or 6 years old, then the brain achieves its complete strength. So, from this, the cognitive development of early childhood has been realized that it is more important that the brain achieves all its weight within 6 years. Along with that, it also achieves its complexity. That is to say, in managing different things, in looking at them logically, in other things. Then, the development that happens later is definitely there along with the time. But, it is very important that the brain achieves all its weight within 6 years. But, it is very less. And, in the beginning, these things are very strong. Now, let us look at the best and most elaborative theory in this series. That is the theory of onion, of cognitive development. Now, we are looking at the sensory motor state of our child who is 2 years old. The child begins to interact with the environment. In the beginning, you must have seen that children are very young, and now they only focus on their faces. When they are a little older, they are attracted to the object. When they are 4 or 5 months old, then they can catch it. After that, gradually, the grip on their things increases, the control increases, and the control of their body also increases. But, during all this, they still use their senses. What they see, what they see, what they observe, they see. But, as soon as the child is 2, 6, or 7 years old, then you see that he goes to the pre-operational stage. Here, the child begins to represent the world symbolically. Earlier, he could not imagine anything. Now, he imagines the symbols as well. For example, if we take a game of 3 months of the child, then you will hide him behind the wheel. He will not look back. If he is a 5-month-old child, then he will look back and see that there is something behind him. Now, when the child is 2 or 7 years old, then he can see all the things symbolically. Then, the concrete operational stage, it goes ahead. In the 7 to 11 years, children learn conservation here. They learn that the subject remains in some state. It can change, but it remains in its state. Now, they can understand the logic, categorize things as well. There are many functions that they can do concretely. The final stage, which is our formal operational stage, you see that children start thinking abstractly. It starts after 12 years. Now, they can create those things without observation, which may not happen before. They can make something new. Extended abstract, which you call abstract, which can only be imagined, they also think about it. Then, they can make some logical relations, they can work on some puzzles, they can work on jigsaws. There are many complex structures, as you must have seen, that after coming here, the operations of complex mathematics and language can be done by children. This will continue during and after becoming an adult. Formal operational stage, in this, our adolescents, after 12 years, now they start learning abstract things. They can solve many puzzles, they can think abstract ideas, mechanically, physically. They can do such abstract things, which can be done by adults also. After this, our brain can do its functions completely. Now, the only difference is the interaction. Some people, in this age, the way they interact, the way they interact with materials, the way they get coursework or content, the way they live in the city, the situation they live in, the way they work, and some people can do better if they live in a better environment. Otherwise, their mental processes, cognitive development, can be completed after this age.