For girls, puberty begins when the specialized hormones tell the ovaries to begin producing another hormone, known as estrogen. Estrogen is responsible for most of the changes in a girl’s body during puberty. It causes her body to mature and prepare for pregnancy. In males, the follicle-stimulating hormone and the luteinizing hormone act on the testicles. The testicles, or testes, are two egg-shaped glands that produce sperm and male hormones.