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.
--------
Question: Sentence1: a fire requires oxygen to burn. 
Sentence2: If the oxygen is removed, the fire is smothered.

Answer: oxygen


Question: Sentence1: the emission of sulfur dioxide causes acid rain. 
Sentence2: the emission of sulfur dioxide causes soil weathering.

Answer: causes


Question: Sentence1: Ice wedging is the most important form of mechanical weathering. 
Sentence2: cycles of freezing and thawing water cause mechanical weathering.

Answer:
mechanical