EDU 307 Free Write - Where did my imagination go?

For My EDU 307 class we need to write an original piece of work. I chose to write two short children's books. When I was younger, I would have been able to come up with two very imaginative and detailed stories very quickly. It would have been no problem to write my own story. It may not have been as good of quality as far as spelling and the plot line, but it would have been very creative. However, the other day when I was sitting in class trying to brainstorm a story, I could not come up with an idea for the life of me. I was going to switch topics and do a memoir or something else, but I decided that I should be able to come up with a story and I should stick to it. So currently, I am still in the process of writing the story, but it made me think, what happened to my imagination? What happened to pretend play? Why is it so difficult to do such simple things now at the age of twenty one, when I could do it with such ease at the age of five? Does your brain just get rid of those abilities or store them away? I am very good at reading and understanding stories now, but I have a lot of difficulty making up my own.   I thought it would be so much easier than it actually is. I am currently being challenged to write a children's book.  And as far as pretend play goes, I don't normally whip out my old barbies and play with them for hours like I used to. I guess as adults you just grow out of the love you used to have for your toys. It reminds me of Toy Story 3 when Andy gives away all of his toys to a younger girl because he is going away to college. It is just crazy to think that at one point in my life all I wanted for Christmas was a new barbie or baby doll and now I want an iPhone. Sometimes I wish that I was five again just so I could have my imagination back.