How does a girl know that she's having her period? And what should she do?

One day, she notices some blood on her knickers. To absorb the flow of blood, girls place a panty liner inside their knickers. They can also insert a tampon into their vagina to absorb the blood before it escapes. Sometimes, girls can have a sore tummy before and during their period. This is caused by the uterus contracting to expel the blood.