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: Bird eggs have hard, calcium carbonate shells. 
Sentence2: The reproductive cells of a bird is a hard, calcium carbonate shell.
Example Output: calcium

Example Input: Sentence1: Some herbivores have more than one stomach. 
Sentence2: Camels are an herbivore.
Example Output: herbivores

Example Input: Sentence1: Turtles can have roundworms. 
Sentence2: Turtles can be infected with nematoda.
Example Output:
Turtles