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: Enveloped viruses make their own membranes. 
Sentence2: Some viruses make their own membranes.
Example Output: make

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: Intrusive igneous rocks account for sections of higher elevation. 
Sentence2: vein deposits of gold and silver are formed in higher elevations.
Example Output:
elevation