Five-year-old children are in a period of transition. Many aspects change for them over the course of this year. Children's brains grow at exceptional rates and, by the end of his or her fifth year, a five-year-old's brain will reach about nine-tenths of its potential adult size. Five-year-olds also begin changing physically: Their bodies begin to slim down and lengthen. A five-year-old's motor coordination is continually improving, especially his or her fine motor skills. In five-year-olds like other areas of growth and maturation, sexual development is a process that appears to be continual throughout an individual's lifetime. Children aged two to six fall into the preoperational stage in cognitive development. Five-year-olds are transitioning from being egocentric, focused more on self, and ruled by physical and emotional impulsivity. Children begin their fifth year emotionally calm and serene. Social skill development is interlinked with relational development for five-year-olds. Five-year-olds try out a triangular relationship first with parents.