* A library is to a book. Both represent collections of individual units (trees in a forest, books in a library).
* A city is to a building. A city is a larger structure made up of many individual buildings, just like a forest is made of many individual trees.
* An orchestra is to a musician. An orchestra is a group of musicians working together, similar to how a forest is a group of trees growing together.
The best answer depends on what aspect of the relationship you want to highlight.
