Generate an overlapping word between the given two sentences. When you find the overlapping words, they don't have to match exactly, e.g., "survival" and "survive" are valid overlapping words. Little words like "the" or "of" don't count! You must generate significant words which are not the stop words.

Sentence1: Sponges are filter feeders. 
Sentence2: Hawksbills feed primarily on filter feeders.
feeders

Sentence1: Echinoderms lack respiratory and excretory systems. 
Sentence2: echinoids lack a respiratory system.
respiratory

Sentence1: a fire requires oxygen to burn. 
Sentence2: Fires need to be in contact with the air to burn.

burn