You know you’re becoming a tween when you start to think about your body in a new way, and then your body starts to change.

Before you’re a tween you’re just a kid.  As a kid, you probably didn’t think much about your body except things like when you used it to run, is it strong enough when you fight with your brother or if you feel sick.

As you become a tween you become aware of your body, including how it compares to your friends, and how it feels to start experiencing changes like growing hair under your arms, and getting breasts.

Because girl’s bodies develop according to different timetables, a lot of the comparison of bodies that happens is about being too developed or not as developed as your friends.