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: Distance is measured in meters the SI unit of distance. 
Sentence2: The SI unit of distance is in meters.
Example Output: SI

Example Input: Sentence1: Cytoplasm refers to all of the cellular material inside the plasma membrane. 
Sentence2: All cells have a cytoplasmic membrane.
Example Output:
Cytoplasm