Girls are born with all the eggs they will ever have. An ovary releases one egg about once a month. Before the egg is let go, the uterus begins building up a lining. It is made of tissue and blood like almost everything else inside us. The lining is like a nest for the egg if pregnancy happens.
Ovulation happens when the egg leaves the ovary.
Then, the egg moves through a tube toward the uterus. Most of the time, the egg breaks apart before it gets to the uterus. But that doesn't always happen. If a sperm from a boy's penis meets the egg on its way to the uterus, they can join together. The joining of an egg and a sperm is called fertilization.