To make a baby, a man' s sperm meets and joins with a woman' s egg cell inside her body. Inside the man' s sperm are a set of instructions that tell the baby to be a boy or a girl.
The instructions in the man' s sperm cell can either carry the letter "X" or the letter "Y". If the letter is an "X", it means the baby will be a girl. If the letter is a "Y", the baby will be a boy.