The first sign of puberty for boys, which usually occurs between ages 10 and 14, is an increase in the size of the testicles, also referred to as your gonads. These sex glands, housed in a sac called the scrotum, wake up during early adolescence and start to pump out testosterone in response to chemical messages from your brain’s hypothalamus and pituitary glands. Without any warning, you may find yourself shifting from thinking about trucks to thinking about sex. Testosterone is also responsible for your developing a beard and other body hair, your deepening voice, and your penis widening and lengthening. On the negative side, this hormone is the culprit behind a teen’s breaking out with zits at the most inopportune times. It can even make you do stupid things just to impress your latest love object.