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: boats 
Words: lifeline, stern, fold, deck, rudder
fold

Category: mode of transportation 
Words: train, spider, wave, fly, drive
spider

Category: aesthetic characteristic 
Words: level, fair, milky, size, make the grade

make the grade