A cool story by biologist Dr. Stuart Firestein, from the book This Will Make You Smarter:
"Instinct refers to a set of behaviors whose actual cause we don't know or simply don't understand or have access to; and therefore we call them instinctual, inborn, innate. Often this is the end of the exploration of these behaviors, they are the nature part of the nature-nurture argument [...] and therefore can't be broken down or reduced any further. But experience has shown that this is rarely the truth. In one of the great examples of this, it was for quite some time thought that when chickens hatched and they immediately began pecking the ground for food, this behavior must have been instinctive.
In the 1920s a Chinese researcher named Zing-Yang Kuo made a remarkable set of observations on the developing chick egg that overturned this idea — and many similar ones. Using a technique of elegant simplicity he found that rubbing heated Vaseline on a chicken egg caused it to become transparent enough to see the embryo inside without disturbing it. In this way he was able to make detailed observations of the development of the embryo from fertilization to hatching. One of his observations was that, in order for the growing embryo to fit properly in the egg, the neck is bent over the chest of the body in such a way that the head rests upon the chest just where the developing heart is encased. As the heart begins beating the head of the chicken is moved in an up-and-down manner that precisely mimics the movement that will be used later for pecking the ground.
Thus the "innate" pecking behavior that the chicken appears to know miraculously upon birth has, in fact, been practiced for more than a week within the egg."
The book's contents are all available online.