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.
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Question: Sentence1: a fire requires oxygen to burn. 
Sentence2: A fire requires something with exactly eight protons to burn.

Answer: burn


Question: Sentence1: Acids dissolve in base and turn blue litmus red. 
Sentence2: acid changes color.

Answer: Acids


Question: Sentence1: Roots gather plant nutrients. 
Sentence2: looseness of soil has a positive impact on plant nutrients.

Answer:
nutrients