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: Cells are the units of function in living things. 
Sentence2: living things cannot survive without oxygen.

A: living
****
Q: Sentence1: Energy enters ecosystems in the form of sunlight or chemical compounds. 
Sentence2: Energy enters ecosystems in the form of photons.

A: Energy
****
Q: Sentence1: triceps contracting causes a person 's elbow to straighten. 
Sentence2: Muscles in the upper arm contracting causes a person 's elbow to straighten.

A:
causes
****