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.

Example Input: Sentence1: threatening behavior by predators causes a porcupine to extend its quills. 
Sentence2: Fishers are one of the few specialized predators on porcupines.
Example Output: predators

Example Input: Sentence1: healing requires rest. 
Sentence2: Healing and cell regeneration follows.
Example Output: healing

Example Input: Sentence1: Flowing water can slowly dissolve carbon in sedimentary rock. 
Sentence2: flowing water can slowly dissolve carbon in limestone.
Example Output:
Flowing