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: Plants exhibit phototropism, or growing toward a light source. 
Sentence2: Bamboos are vigorous, rugged plants.
Example Output: Plants

Example Input: Sentence1: melting is when solids are heated above their melting point. 
Sentence2: Ice is a solid, it's cold and it's hard.
Example Output: solids

Example Input: Sentence1: air pollution can cause the pH of soil to decrease. 
Sentence2: Acidity increases as the pH number decreases, and alkalinity increases as the pH number increases.
Example Output:
decrease