Functional fixedness is described as a type of:

1. The obstacle comes from a fixed way of approaching the problem. 2. Functional fixedness is a type of mental set. 3. A mental set is a habitual way of framing a situation. 4. In functional fixedness that framing is tied to an object's designed purpose. 5. It is a problem-solving obstacle rather than a failure of memory or perception. _Source: OpenStax Psychology 2e (CC BY 4.0), Ch 7 'Thinking and Intelligence', sections 7.1-7.5_