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 Q]: Sentence1: triceps contracting causes a person 's elbow to straighten. 
Sentence2: muscles contracting cause a person's elbow to straighten.
[EX A]: contracting

[EX Q]: Sentence1: Cells are the units of function in living things. 
Sentence2: living things cannot survive without oxygen.
[EX A]: living

[EX Q]: Sentence1: cutting down trees has a negative impact on an ecosystem. 
Sentence2: Cutting down oaks has a negative impact on an ecosystem.
[EX A]:
a