Duncker's candle problem is a classic experiment used to demonstrate functional fixedness. In this task, participants receive a candle, a book of matches, and a box of thumbtacks, and are asked to attach the candle to a wall without allowing wax to drip onto the floor. To solve the problem, one must overcome the tendency to view the box solely as a container for thumbtacks, and instead empty it, tack it to the wall to serve as a shelf, and stand the candle inside.