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: a fire requires oxygen to burn. 
Sentence2: A fire requires something with exactly eight protons to burn.
Example Output: burn

Example Input: Sentence1: Fertilization occurs naturally in the fallopian tubes. 
Sentence2: fertilization is used for reproduction.
Example Output: Fertilization

Example Input: Sentence1: Insects can spread disease and destroy crops. 
Sentence2: pests destroy crops.
Example Output:
crops