Most of the biological changes that occur in our lives happen in the period of adolescence. Changes involved are both physical and psychological, and vary greatly depending on biological sex and external environment. The first and most noticeable of these change is a relatively rapid change in body shape and height, commonly referred to as a "growth spurt." Height changes in the midst of adolescence may be even more rapid and abrupt than it is over the course of the rest of the individual's life combined. Psychologically, oxytocin regulation within the body also changes rapidly, greatly altering the psychological means by which adolescents interact with themselves and others.
Perhaps the most daunting of these changes is the development of secondary sex characteristics, commonly referred to as puberty. For biological males, this entails the development of genitals, including the onset of erections and ejaculation. For biological females, this includes the enlarging of breasts as well as the onset of menstruation, or the expulsion of unnecessary tissue from the lining of the uterine wall, which is colloquially known as a "period." Some aspects of puberty are universal to both sexes, including the development of body hair and the onset of an interest in sexuality.