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 flashlight converts chemical energy into light energy. 
Sentence2: PALight Flashlights are the ultimate personal LED flashlights.
Example Output: flashlight

Example Input: Sentence1: Carpools and vanpools can save time. 
Sentence2: vanpools decrease the amount of cars used to travel to a place.
Example Output: vanpools

Example Input: Sentence1: treating water is used to remove harmful substances before drinking. 
Sentence2: treating water is used to remove chlorine, pesticides, and heavy metals before drinking.
Example Output:
before