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: an animal requires air for survival. 
Sentence2: It is breathed in by animals and restored to the air by the photosynthesis mechanism of plants.

Answer: air


Question: Sentence1: Plants live just about everywhere on Earth. 
Sentence2: epiphytes live everywhere on earth.

Answer: everywhere


Question: Sentence1: When a gas expands, it cools off. 
Sentence2: If a gas is heated then that gas will cool off.

Answer:
cools