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: DNA is made up of nucleotides. 
Sentence2: Mutations can change the structure of DNA.
Example Output: DNA

Example Input: Sentence1: Most water comes from precipitation. 
Sentence2: aquatic animals require precipitation to survive.
Example Output: precipitation

Example Input: Sentence1: protein is used to repair cells by the human body. 
Sentence2: Protein Protein is needed for tissue maintenance and repair.
Example Output:
repair