The changes during puberty take place slowly over months and years. For example, facial hair growth in boys starts with a few scattered hairs and gradually becomes more uniform.

These changes do not occur at the same time or at a specific age for everyone. The onset and pace can vary widely among individuals.

Boys develop new thick hair on the face, experience voice cracking, and may witness occasional enlargement and erection of the penis, often in daydreams or at night.

Girls experience an increase in breast size and darkening of the skin of the nipples. Menstruation also begins during this period.

Sexual reproduction, by combining germ cells from two individuals during fertilization, ensures the reestablishment of the proper number of chromosomes and DNA content in the new generation. This process helps maintain genetic stability and control over cellular functions.