Final answer:
Millie's fixation on the empty plastic milk jug not being useful anymore is an example of functional fixedness.
Step-by-step explanation:
Millie's fixation on the empty plastic milk jug not being useful anymore is an example of functional fixedness. Functional fixedness is the inability to see an object as being useful for something other than its intended purpose. In this case, Millie is fixated on the milk jug only being useful for holding milk, ignoring the potential for reusing or repurposing it for other purposes.