What's in a yolk?

I made boiled eggs for my girls tonight. They love hard boiled eggs, they always have. Sophie and Liv both finished peeling their eggs and began discussing which part they liked better, the yolk, or the egg white. Liv likes the egg white and Sophie likes the yolk, which is funny, because if you had this discussion with them a year ago their favorite part of the egg was reversed, Liv liked the yolk and Sophie liked the egg white. Anyway, Sophie had eaten all of her egg white and was down to the best part, the yolk. She came up to me and said, "MMMM, I love this fluffy yellow stuff. Mom, what do they put in the yolk to make it taste so good?" It was the cutest thing I have ever heard. I tried to explain that it was an unfertilized chicken egg, but I confused the hell out of her. By the end of our conversation, we agreed that she was eating a baby chick, but not the cute fluffy kind... LOL!