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.

Sentence1: creating paper requires cutting down trees. 
Sentence2: Trees begin as epiphytes on host trees.
trees

Sentence1: All life relies on a relatively narrow range of pH, or acidity. 
Sentence2: humans rely on a relatively narrow range of acidity.
acidity

Sentence1: Another common pollutant is airborne particulate matter. 
Sentence2: Smog is an airborne pollutant.

airborne