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: Rivers, rainwater, well water and most lakes contain fresh water. 
Sentence2: canyons are formed by rainwater.

Answer: rainwater


Question: Sentence1: Erosion of rock makes nutrients available. 
Sentence2: rain causes nutrients to be available.

Answer: nutrients


Question: Sentence1: Any antigen that causes an allergy is called an allergen. 
Sentence2: Any foreign protein that causes an allergic reaction is called an allergen.

Answer:
allergen