Like constructing a house, brains are built upon a strong foundation. This starts before birth and is very important during the first three years of life. Brain cells are "raw" materials — much like lumber is a raw material in building a house, and a child's experiences and interactions help build the structure, put in the wiring, and paint the walls. Heredity (nature) determines the basic number of "neurons" (brain nerve cells) children are born with, and their initial arrangement.
At birth, a baby's brain contains 100 billion neurons, roughly as many nerve cells as there are stars in the Milky Way, and almost all the neurons the brain will ever have. The brain starts forming prenatally, about three weeks after conception. Before birth, the brain produces trillions of more neurons and "synapses" (connections between the brain cells) than it needs. During the first years of life, the brain undergoes a series of extraordinary changes.
In the brain, the neurons are there at birth, as well as some synapses. As the neurons mature, more and more synapses are made. At birth, the number of synapses per neuron is 2,500, but by age two or three, it's about 15,000 per neuron. The brain eliminates connections that are seldom or never used, which is a normal part of brain development.
"Windows of opportunity" are sensitive periods in children's lives when specific types of learning take place. For instance, scientists have determined that the neurons for vision begin sending messages back and forth rapidly at 2 to 4 months of age, peaking in intensity at 8 months. It is no coincidence that babies begin to take notice of the world during this period.