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: physical property 
Words: protein, pasty, fat, thin, angular momentum
protein

Category: moral characteristic 
Words: mild, equitable, unprejudiced, trustworthy, fair
mild

Category: animals 
Words: duck, cat, stoop, swan, goose

stoop