Functional fixedness is the tendency to think only of an object's most common use in solving a problem. Example: Rachel's car breaks down while she is driving through the desert. She is terribly thirsty. She finds several soda bottles in the trunk but no bottle opener. She doesn't think of using the car key to open the bottles because of functional fixedness.
Mental Set
A mental set is a tendency to use only those solutions that have worked in the past.