By the age of 11, childhood is almost behind us as we stand on the edge of adulthood. We’re about to go through changes like never before, thanks to the hormone surge known as puberty. Puberty doesn’t follow a strict schedule; it can begin at different times based on our DNA, lifestyle, and even our diet. Whether you’re a boy or a girl, puberty starts in the brain, at the base of the hypothalamus—the same area that controls body temperature.

Puberty kicks off when the hypothalamus releases a protein called kisspeptin into the brain. This protein sets off a chain reaction of releasing other hormones throughout the body, spurring the sex organs to mature. For girls, this means they will experience ovulation. Unique footage from a fertility center shows an egg inside a protective fluid. When ovulation occurs, the egg travels down the fallopian tube, where it will either be fertilized or lost through menstruation.

Starting with puberty, ovulation happens once a month. But the ovaries take on another crucial role—they begin releasing hormones into the bloodstream, including estrogen. These hormones have a dramatic impact on both emotional and physical development. Boys and girls both experience growth spurts. Day by day, as we look in the mirror, we see a face that seems to change overnight.

Girls transform into women, and boys into men. For boys, the brain also releases kisspeptin, leading to a surge in testosterone produced by the testicles. This hormone causes the larynx to enlarge and the vocal cords to lengthen, resulting in a deeper voice. Testosterone also stimulates muscle growth and the development of body hair.

But it’s not just our outward appearance that changes. Inside, our brains undergo significant rewiring, affecting mood and character. Both boys and girls experience a flood of new emotions, one of which is new and overpowering—attraction to the opposite sex. Our bodies react dramatically: heart rates speed up, blood pressure rises, and we may feel flushed or excited.

During adolescence, many experience the milestone of a first kiss. By the end of these transformative years, the statistics are staggering: we grow nearly four times our original height, become 21 times heavier, and have digested nearly nine tons of food. Our hearts have beaten over a billion times, and our lungs have taken around 200 million breaths. Finally, we’re ready, both mentally and physically, to step into adulthood.