(Well, I started writing a book. Then I asked the boys to help me.)
The title of my book was Jacob's Bear. It begins with a young boy who goes to bed and while lying in the dark, he hears a growling in his closet. He goes into his closet to investigate and slowly his closet turns into a forest...

I never said it was original. At the time, Jacob was afraid of bears, so much so that he was anxious about camping several months away. My goal was to write a story to educate him about bears and ease his worries.
Crack!
...he held himself perfectly still. There are many frightening creatures hidden in these trees. Tigers, snakes,
gasp! ba-ba-bears!
Any one of them could have heard the twigs snapping under his feet. Any one of them could pounce at any moment. With a deep breath, he whispered, "I...am...brave." and he trudged on. Bravely. But quietly- just to be on the safe side.
Then one night while putting the boys to bed, I said "Let's make up a story." I thought it was a mistake, but it turned out to be quite an adventure. The story has the same beginning, but now the boy is named James, and he has a pet mouse (that he met inside the volcano) whose name is Wilsom. He has many adventures that I could never dream up on my own.
Here is a sample- James travels to a clearing where he came across a lake. Boom! Boom! What could it be? Cannons? Maybe it's a ... crash! A pirate ship crashes onto the shore.
Jacob "If it crashes onto the shore, how do they get it back into the water?"
Me "They don't. It's too heavy to push back into the water."
Joe "Then how to they get boats into the water after they build it? Oh, I know, they build the bottom part, then they push it in the water, then build the rest in the water."
"How does something so heavy not sink?"
"Can we get back to the story? Ugly, mean pirates crawled off the ship, and they were so hungry..."
"They bended down at an ant pile and started eating ants."
"Were they red ants, or black ants, or yellow ants?"
"Which ones are worser? Red, black, or yellow, which ones do you think?"
As you can see, James doesn't get very far. But I am asked often, "Hey remember that thing we did with the story we made up, remember? Can we do that tonight?"
One night, James was in a pyramid. They knew that pyramids were in Egypt, so they jumped up and found Egypt on their map. "There! He's right there!" So after he escaped from the mummies, he found himself at the Red Sea. Now he's on his way to Madagascar. We have turned our story hour into a geography lesson. James currently is trapped in a Baobab tree in deep in the forest of Madagascar, with his friend, Tojo the cannibal, that he picked up on his way. Hyenas surround the tree. Oh, no! What will he do?
As I tell the boys at the end of our story, "to be continued..."