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: Mutations are essential for evolution to occur. 
Sentence2: Genes are altered for evolution to occur.
Example Output: evolution

Example Input: Sentence1: Nitrogen is found in proteins, nucleic acids, and chlorophyll. 
Sentence2: Lawns eat nitrogen.
Example Output: Nitrogen

Example Input: Sentence1: Air contains oxygen. 
Sentence2: air helps cells survive.
Example Output:
Air