I buy puzzles that relate to the subject I am teaching. For example if I'm teaching World Geography, I buy a map of the world. I keep one puzzle out at a time and tell the students they may only work on the puzzle when all their work is complete. I also tell the students that I am going to glue the puzzle together and display it in the classroom when they're finished. I have students who rarely finish an assignment, finish their work in order to work on the puzzle. I have yet to have any pieces come up missing. All the kids want to see the puzzle hung up in the room when it is finished. I write the month and year the puzzle was completed and hang the puzzles up all year instead of posters.