instruction:
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.
question:
Sentence1: gills are used for breathing water by aquatic animals. 
Sentence2: gills are used for breathing water by sharks.
answer:
are


question:
Sentence1: With a spatula, fold in flour mixture. 
Sentence2: a sifter is used for separating flour.
answer:
flour


question:
Sentence1: Bottleneck effect occurs when a population suddenly gets much smaller. 
Sentence2: Animals and plants exist as populations.
answer:

population