ELI5: Explain Like I'm 5

Fertilisation

Fertilisation is when two cells, called sperm and an egg, join together to make a baby. When a man and a woman want to make a baby, the man's sperm joins with the woman's egg inside her body. The sperm and egg each have half the information needed to make a baby, so when they join they make one cell with all the information it needs to create a baby. This baby is known as an embryo.