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.

Ex Input:
Sentence1: cell division often causes growth. 
Sentence2: Mitosis is cell division.

Ex Output:
cell


Ex Input:
Sentence1: Finally, the fluid in the nephron reaches a collecting duct . 
Sentence2: fluid in the kidneys collect into ducts.

Ex Output:
fluid


Ex Input:
Sentence1: Tree rings are examined to learn the forest age and growth rate. 
Sentence2: growth rate can be estimated with tree rings.

Ex Output:

growth