A “puzzle elf” is a term used within the puzzle community to describe someone who goes out and leaves puzzles for other people to find. They leave these delightful gifts in public spaces for strangers to pick up and enjoy.
More specifically, a book elf:

• Is part of a community. The term originated in the book community and is associated with a specific, and often seasonal, trend of spreading kindness through sharing puzzles.

• Performs a simple, kind deed. The puzzle elf anonymously shares a puzzle with a stranger simply for the pleasure of passing on a good puzzle.

• Operates on a “take one, leave one” principle. In some variations, the tradition is that the finder will complete the puzzle and then “re-release” it somewhere else for another person to find.

• Often includes a note. Sometimes, the puzzle has a note or a sticker inside to explain the purpose of the gift and encourage the recipient to pass it on.

• Enthusiasm for puzzling: Their passion for puzzles makes the experience of finding a new story enjoyable for others. This term is a colloquial and informal expression. In general, it refers to a lover of puzzles or an assistant who helps with puzzles.

This term is a colloquial and informal expression. In general, it refers to a lover of puzzles or an assistant who helps with puzzles.