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: Combustion-related, motor vehicles, wood burning, open burning, industrial processes. 
Sentence2: Combustion is used to produce heat.

Answer: Combustion-related


Question: Sentence1: An organism can gain energy by assimilation or absorption of food. 
Sentence2: Organisms require food for growth.

Answer: food


Question: Sentence1: Each species lasts for a time, sometimes surviving to the present and sometimes going extinct. 
Sentence2: Population size influences the chances of going extinct.

Answer:
extinct