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.

Category: type of sandwich 
Words: picnic, open-faced, club, grilled cheese, lettuce
picnic

Category: employment activity 
Words: recruit, replace, sack, heat, fire
heat

Category: employment activity 
Words: ban, fire, empower, sack, ground

ground