Given a category and a set of five words, find the word from the set that does not belong (i.e. is the least relevant) with the other words in the category. Words are separated by commas.

[Q]: Category: temperature 
Words: cold, hot, cool, popular, relaxing
[A]: popular


[Q]: Category: window material 
Words: glass, coating, panel, spatula, lead
[A]: spatula


[Q]: Category: guitar part 
Words: frustrated, headstock, notes, strap lock, fret
[A]:
frustrated