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: Ferns are vascular plants. 
Sentence2: when a fern grows, its size of leaves may increase.

Answer: Ferns


Question: Sentence1: Mating is generally preceded by courtship. 
Sentence2: Mating is generally similar across species.

Answer: Mating


Question: Sentence1: Some arthropods have special excretory structures. 
Sentence2: Crustaceans are, like insects, arthropods.

Answer:
arthropods