So imagine your mom or dad is writing a big list of all the stories they have heard from their grandparents, great-grandparents, and so on. They write down everything, from the time when the world was created, to the stories about Abraham, Jacob, Joseph, and all the other people from Bible times.
Now imagine that list gets passed down through generations, to your great-great-great-great grandparents, and so on. They keep adding stories that they hear from their own families, but they also start removing some parts that they think might not be important or true.
After many years, the list gets really long and people start wondering which stories should be kept and which ones should be scrapped. So a group of very wise and learned people gather together to discuss and decide which stories should be included in the list of important stories, which we call the Old Testament.
They talk about it a lot, do a lot of thinking and praying, and finally come up with a list of stories that they believe are the most important ones. These stories are then written in a special book, which becomes the Old Testament that we know and read today.
However, not everyone agrees with this list of stories. Some people have different ideas about which stories should be kept and which ones should be removed. So there are different versions of the Old Testament in different parts of the world, but the one that most people read are the stories chosen by those wise and learned people long ago.