Question: Why do girls go on their periods? I know its because our eggs come out but why do our eggs come out ? Why dont they just stay inside us?

Hayley Evers-King answered on 20 Mar 2013:

Scientists think its actually more to do with shedding the lining of the uterus than the eggs themselves. Though the reasons aren’t entirely certain. Some scientists have suggested its more energy efficient to shed the lining than reabsorb it and maintain it. Did you know chimpanzees have periods just like ours too? Something else that shows how close out ancestry is.