To complete this silly task, you can take the tacks out of their box, tack the box onto the wall, and place the lit candle inside the box. Ta-da! Did you solve it?
Duncker noted that this problem requires participants to overcome their tendency for “functional fixedness.” He described that as a “mental block against using an object in a new way that is required to solve a problem.” In this problem, the tack box is key. Participants had a hard time seeing the box in any way other than a thing containing tacks. A variation of this problem had the tacks presented to the participants outside the box, and the participants were much more likely to figure out the tack-box-to-wall strategy. The moral of Duncker’s research? Think outside the box, literally, and don’t be too tied to the usual functions of familiar objects.