Word study's are learning about words - not memorizing them from a word list. It is a method of investigation, discussion and active learning.
Nouns with an 'y' on the end can change to plural by dropping the 'y' and adding 'ies'. Instead of merely telling your students the rule and expecting them to remember it, by discussing the topic and giving them prompts and hints they may arrive at this answer themselves.
This is far more meaningful than lazily writing out the rule for them to remember (Bush, 2008).