A baby’s brain is not fully developed until the child is a teenager. The brain starts developing as soon as the baby starts developing in the womb. At some points the baby’s brain is making new cells at a rate of 250,000 per minute! Once the baby is born the brain still not fully developed and continues to grow until we become adults.
There are lots of different types of brain cell. Some which allow the brain to make your muscles work can stretch from your spine all the way to your toes. Others are so small that thousands of them would fit in the 1mm space in your ruler. But in general cells in your brain are between 0.001mm and 0.1mm in width.
Did you know that there are as many cells in your brain as there are stars in the universe?