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: Energy enters ecosystems in the form of sunlight or chemical compounds. 
Sentence2: Energy enters ecosystems in the form of photons.
Example Output: Energy

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

Example Input: Sentence1: Nephrons are the structural and functional units of the kidneys. 
Sentence2: nephrons are the structural and functional units of the organs that manufacture urine.
Example Output:
Nephrons