The Wonderful Adventures of Nils (orig. Nils Holgerssons underbara resa genom Sverige) is a famous work of fiction by the Swedish author Selma Lagerlöf.
The book is about a young lad, Nils Holgersson, whose "chief delight was to eat and sleep, and after that he liked best to make mischief". He takes great delight in hurting the animals in his family farm. Nils captures a tomte in a net while his family is at church and have left him home to memorize chapters from the Bible. The tomte proposes to Nils that if Nils frees him, the tomte will give him a huge gold coin. Nils rejects the offer and the tomte turns Nils into a tomte, which leaves him shrunken and able to talk with animals, who are thrilled to see the boy reduced to their size and are angry and hungry for revenge. While this is happening, wild geese are flying over the farm on one of their migrations, and a white farm goose attempts to join the wild ones. In an attempt to salvage something before his family returns, Nils holds on to the bird's neck as it successfully takes off and joins the wild birds.
The wild geese, who are not pleased at all to be joined by a boy and a domestic goose, eventually take him on an adventurous trip across all the historical provinces of Sweden observing in passing their natural characteristics and economic resources. At the same time the characters and situations he encounters make him a man: the domestic goose needs to prove his ability to fly like the experienced wild geese, and Nils needs to prove to the geese that he would be a useful companion, despite their initial misgivings. During the trip, Nils learns that if he proves he has changed for the better, the tomte might be disposed to change him back to his normal size.