Why are there significant changes in the body at puberty stage?

Puberty is made up of a clear sequence of stages, affecting the skeletal, muscular, reproductive, and nearly all other bodily systems. Physical changes during puberty tend to be more gradual and steady. This is comforting to many parents who feel childhood passes much too quickly.

Physical:

voice drop,

pubic hair,

increase in height,

weight gain (muscle development)