P.E.R.I.O.D.

Period.

Why do girls have this?

Scientifically, period is often called as Menstruation. Menstruation is the periodic discharge of blood and mucosal tissue (the endometrium) from the uterus and vagina. It starts at menarche at or before sexual maturity (maturation), in females of certain mammalian species, and ceases at or near menopause (commonly considered the end of a female's reproductive life). The periodicity of menstruation gives rise to commonly used euphemisms like "period" and "monthly".

Let's start this weird journey ^___^