First of all, let me clarify if you are indeed a girl, and notsome nosy boy. Assuming the latter, a girls period is when her bodyis preparing for having children one day. When a girl gets herperiod on the onset ofpuberty, she is capable of getting pregnant. The body releasesan egg in her ovaries and the egg travels down the Fallopian tubeinto her uterus. To prepare for possible fertilization of that egg(sperm released during sex that, when meets with an egg in hervagina, creates a baby), the uterus builds up a lining of blood andtissue. If the egg is not fertilized within a few days, thebody will shed that lining through and out agirls vagina, completing the menstrual cycle. Girls get periodsonce a month, normally at the same time each month, and they canlastanywhere from 3-7 days.