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: Complex chemistry is needed to carry out all the functions of life. 
Sentence2: Chemistry is primarily concerned with matter, energy, and their interactions.

Answer: chemistry


Question: Sentence1: lightning can cause a forest fire. 
Sentence2: Lightning can cause a natural cycle in forests.

Answer: lightning


Question: Sentence1: Active immunity results when an immune response to a pathogen produces memory cells. 
Sentence2: vaccination results when an immune response to a pathogen produces memory cells.

Answer:
cells