Their testicles get bigger and hang lower. Often one hangs lower than the other. Their penises grow larger. Their penises also get hard more often and unexpectedly. Boys usually start growing beards after their voices change.
Beards grow very slowly for the first couple of years.
Sometimes boys' breasts become tender and large, too. Usually this lasts only a year or two, and then their breast size returns to normal again. These hormonal changes also can cause abrupt changes in our moods. Often puberty and adolescence make us feel like we're on an emotional roller coaster. Whether we grow into large or small adults has a lot to do with what our parents are like.