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.

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

A: acidity
****
Q: Sentence1: plant requires seed dispersal for reproduction. 
Sentence2: plants are pollinated by birds.

A: plant
****
Q: Sentence1: Some pelycosaurs gave rise to a group of animals called therapsids. 
Sentence2: Intelligent, evil therapsids invade earth.

A:
therapsids
****