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.

Ex Input:
Sentence1: Gravity is a force that pulls on things. 
Sentence2: A force that pulls on things pulls objects towards planets.

Ex Output:
force


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

Ex Output:
respiratory


Ex Input:
Sentence1: the force of water against the seed coat causes a root to emerge from a seed. 
Sentence2: Instead, trees use a method called osmosis to force water upwards.

Ex Output:

force