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

Example Input: Sentence1: a battery converts chemical energy into electrical energy. 
Sentence2: Battery electricity comes conversion.
Example Output: battery

Example Input: Sentence1: Heterotrophs get food by eating other living things. 
Sentence2: fish eat other living things.
Example Output:
living