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: cold fronts cause thunderstorms as they pass by. 
Sentence2: Cold fronts cause lightning.

Answer: cold


Question: Sentence1: Streams may start with runoff or water seeping out of a spring. 
Sentence2: streams may start with runoff or h2o seeping out of a spring.

Answer: Streams


Question: Sentence1: Viral pneumonia is usually milder than bacterial pneumonia. 
Sentence2: Viral and bacterial disease can both be caused by injury of the lungs.

Answer:
Viral