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: Acid rain is precipitation which pH is 5.7 or lower. 
Sentence2: gases from burning coal causes precipitation which pH is 5.7 or lower.
5

Sentence1: Cells are programmed to divide only a set number of times. 
Sentence2: Cells reproduce asexually, by dividing.
divide

Sentence1: a greenhouse is used to protect plants by keeping them warm. 
Sentence2: Hobby greenhouses used to be small glass houses or fiberglass buildings.

greenhouse