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: heating an egg causes a chemical reaction in the egg. 
Sentence2: Insects hatch from eggs.

Answer: egg


Question: Sentence1: Clams are mollusks that live in the sea or in freshwater. 
Sentence2: clams live in most terrestrial, freshwater, and marine habitats.

Answer:
Clams