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.

[Q]: Sentence1: threatening behavior by predators causes a porcupine to extend its quills. 
Sentence2: Fishers are predators that can eat porcupines.
[A]: predators


[Q]: Sentence1: protein is used to repair cells by the human body. 
Sentence2: Wound healing requires protein.
[A]: protein


[Q]: Sentence1: Ethologists usually study how animals behave in their natural environment. 
Sentence2: Ethologists study animal behavior.
[A]:
Ethologists