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: a polar bear requires a cold environment. 
Sentence2: polar bears live where there are fewer animals.
bear

Sentence1: triceps contracting causes a person 's elbow to straighten. 
Sentence2: muscles contracting cause a person's elbow to straighten.
contracting

Sentence1: An antibody binds to an antigen . 
Sentence2: antibodies binds to molecules.

antibody