This is a period of transition for teenagers and there are many crucial milestones. Socially and emotionally, teens worry that they may not be developing at the same rate as their peers. They become extremely self-conscious and may be overly sensitive about their appearance. Teens may rebel against their parents but are also more able to accept the consequences of their actions. This is also a period of enormous physical change and adolescents experience changes in their physical development at a rate unparalleled since infancy. These changes include significant gains in height and weight. Within a year, boys and girls can gain an average of 4.1 inches and 3.5 inches in height respectively. This growth spurt typically occurs two years earlier for girls than for boys and can tend to make both sexes go through a clumsy phase. In terms of their cognitive development, adolescents have greater reasoning skills and have developed the ability to think logically and hypothetically. They are also able to discuss more abstract concepts. They should also have developed strategies to help them study.