Okay, kiddo, let me explain about the Secret Gospel of Mark.
You know how we have the Bible, that big book that tells us about God and Jesus? Well, in the 1950s, some ancient writings were discovered that were like pieces of the Bible, called manuscripts. One of these manuscripts was called the Secret Gospel of Mark.
Now, Mark was one of the guys who wrote parts of the Bible, and his book is called the Gospel of Mark. But this secret one was different because it had some extra parts in it that people hadn't seen before.
Some people think that these extra parts were added later by someone who wanted to change the original story. They might have added things to make Jesus seem even more special, or to make the story more exciting.
But other people think that these extra parts were actually written by Mark himself, and that they just got lost over time.
Either way, the Secret Gospel of Mark is still a mystery, because we don't know for sure where it came from, or why it was kept secret. But it's interesting to think about, and it helps us understand more about the people who wrote the Bible a long, long time ago.