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: boxing terminology 
Words: caution, hearts, canvas, deck, fight
hearts

Category: animal 
Words: fly, dangle, bee, jump, flap
flap

Category: window material 
Words: knive, lead, spyhole, plastic, glass

knive