ELI5: Explain Like I'm 5

Menstruation

Menstruation is when a girl's body starts to prepare for making a baby. Every month, the uterus (the womb where the baby can grow) gets thicker to create a safe place for the baby. It does this by releasing a special kind of fluid called period blood. This fluid flows out of the vagina which is the place from where the baby will come out if the girl gets pregnant.
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