English: The post office box analogy for a linked list. Suppose Alice is a spy who wishes to give a codebook to Bob by putting it in a post office box and then giving him the key. However, the book is too thick to fit in a single post office box, so instead she divides the book into two halves and purchases two post office boxes. In the first box (box 201), she puts the first half of the book and a key to the second box, and in the second box (box 102) she puts the second half of the book. She then gives Bob (bottom) a key to the first box. Derived from public domain works: File:Paperback book black gal.svg, File:Public key shared secret.svg, File:Emblem-person-blue.svg.