Why Can You Not See That the Box Is Also a Shelf? You Already Know What It Is For

Participants are given a candle, a book of matches and a box of drawing pins and asked to fix the candle to a wall so it burns without dripping wax on the floor. The solution is to empty the box, pin it to the wall and stand the candle in it, using the box as a shelf. Most people fail, because the box was presented containing pins and is therefore perceived as a container rather than as a piece of material. Presenting the box empty alongside the loose pins raises the success rate dramatically.

Several findings sharpen the picture:

• •Showing an object performing its usual function strengthens the block
• •Labelling an object by its function strengthens it further
• •Time pressure and stress both make it worse
• •Young children show it much less than adults do
• •So do people unfamiliar with the object's conventional use
• •Describing an object by its parts and materials weakens it

The effect is a cost of something otherwise valuable, which is what makes it interesting rather than merely a mistake. Recognising an object and immediately retrieving what it is for is enormously efficient and is how anybody functions in a world full of objects. That retrieval happens automatically and it suppresses consideration of the object's other properties, since attending to everything a hammer could be would be paralysing. The block is therefore a side effect of expertise, which is also why children, who have weaker associations, are better at these particular puzzles than adults.

Two neighbouring effects limit problem solving in similar ways and are worth telling apart. A mental set is a tendency to keep applying a method that worked before, even after a simpler route becomes available, and it is demonstrated by giving people a series of problems solvable one way and then one that has a far easier solution, which most miss. Design fixation is the tendency of designers shown an example solution to produce variations of it rather than genuine alternatives. Both share the structure of the effect here, namely that prior experience narrows what is considered before any deliberate thinking begins.

Several deliberate techniques reliably reduce the effect and they all work by forcing attention back to properties. One structured method has people break an object into parts, describe each part by what it is rather than what it does, and repeat until the descriptions carry no function words, which has been shown to improve performance on these problems substantially. Listing materials, weights and shapes does the same informally. Bringing in somebody unfamiliar with the domain works because they lack the association. And separating the description of a problem from any mention of the tools available prevents the block forming at all.

Perceiving an object by its usual function suppresses its other properties, which is why people fail to see a pin box as a shelf when it arrives full of pins. The block is a side effect of efficient recognition, so experts and adults suffer more than novices and children. Describing objects by parts and materials rather than uses reliably reduces it.