"To grow a brain" means to develop intelligence or common sense. It refers to the process of gaining knowledge and critical thinking skills. Full definition
The technology, for instance, helped restore the brain function in a mouse that suffered a stroke by growing brain cells on the animal's skin. (techtimes.com)
The research also presents an opportunity to greatly improve techniques for growing brain circuits in a dish that reflect the true diversity of the human brain, they said. (ucsf.edu)
Your child is going to want to experience everything that their rapidly growing brain can conceive of, but won't yet be physically able to do so which can be very frustrating. (mommyedition.com)