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: Female monotremes lack a uterus and vagina. 
Sentence2: Babies are normally delivered from the uterus through the vagina.
Example Output: uterus

Example Input: Sentence1: an animal requires warmth for survival. 
Sentence2: Light and heat and warmth are basic necessities for survival.
Example Output: warmth

Example Input: Sentence1: Among mollusks, the golden snail is a survivalist. 
Sentence2: Most snails have shells.
Example Output:
snail