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: Finally, the fluid in the nephron reaches a collecting duct . 
Sentence2: fluid in the kidneys collect into ducts.
Example Output: fluid

Example Input: Sentence1: if soil is permeable then water easily flows through that soil. 
Sentence2: The increase in permeability is caused by the formation and expansion of ice in the soil.
Example Output: permeable

Example Input: Sentence1: gases from burning coal and oil dissolving in water in the atmosphere causes acid rain. 
Sentence2: power stations which generate electricity produce gases in the atmosphere cause acid rain.
Example Output:
acid