I saw this cool idea and had to blog about it! Each morning, the teacher posts a clue to a the mystery noun and reveals the noun on Friday. Participation is voluntary, but most kids are pretty motivated by the mystery aspect if this activity. The teacher uses a book from the Daily Detectives series by Daryl Vriesenga.  It would also be pretty easy to make your own clues to introduce an important noun in a lesson the following week or clues that describe something your class learned a few months ago as a review. I liked this idea because it reinforces what nouns are in a painless way.