Puberty is the phase of development that begins the sexual processes in boys and girls. In girls, puberty triggers the beginning of the menstrual cycle, and the development of the breasts. In boys, puberty causes the penis and testicles to grow in size, and signals the beginning of nocturnal emissions and sperm production. In both girls and boys, puberty causes body hair (and facial hair, in boys) to begin appearing. During puberty, hair will start to grow under the armpits, in the groin, and on the chest and back. Leg and arm hair will also become thicker and darker.