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: the uterus is used in reproduction. 
Sentence2: Reproduction uses the same organ as menstruation.
Example Output: reproduction

Example Input: Sentence1: when a body of water receives more water than it can hold , a flood occurs. 
Sentence2: Floods are deadly, floods are devastating.
Example Output: flood

Example Input: Sentence1: Heat production is increased by shivering and non-shivering thermogenesis . 
Sentence2: Heat production is increased when many muscles contract a little bit, all at once.
Example Output:
Heat