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: animal 
Words: walk, flap, fish, fly, bats
flap

Category: physical property 
Words: brittleness, large, fat, area, calcium
calcium

Category: outdoor sport activity 
Words: row, fish, iguana, hunting, hunt

iguana