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: air pollution can cause the pH of soil to decrease. 
Sentence2: Acidity is measured with a pH meter or with pH paper strips.
Example Output: pH

Example Input: Sentence1: Eggs are usually laid in a nest. 
Sentence2: birds have nests.
Example Output: nest

Example Input: Sentence1: Water fills the oceans of the world up to sea-level making it a liquid, a type of fluid. 
Sentence2: Animals require a type of fluid to survive.
Example Output:
fluid