Functional fixedness is the tendency to see an object only in terms of its customary function. The mental set prevents the object from being used in a novel way that would solve a problem.

In the candle problem, participants are given a candle, a box of matches, and thumbtacks, and are asked to mount the candle on a wall. Many fail to see that the matchbox can be emptied and tacked to the wall as a platform. The box is fixed in their minds as a container. Similar obstacles appear whenever a tool or everyday object must be redeployed for an unintended purpose.

Functional fixedness is stronger when the object’s usual function is demonstrated just before the problem, and weaker when the object is presented without its typical context. Expertise can both help and hinder: deep knowledge of an object’s properties can suggest new uses, yet strong association with the standard use can block them. Deliberately listing alternative functions or taking a break can reduce the fixation.

The same knowledge that makes an object useful in ordinary circumstances can make it invisible for extraordinary ones.