Answer:
A standard dimension for sandwiches and rulers
I am the first you ever saw, what greets you every morning and what goes out in the end.
Mostly worn by women or girls, this is sometimes shiny like silver or gold, and sometimes it is made of pearls.
It can be cracked, It can be made, It can be told, it can be played. What is it?
What you become during a workout?
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?