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.

Example Input: Sentence1: Algae have varied life cycles. 
Sentence2: And fish eat algae.
Example Output: Algae

Example Input: Sentence1: Very often their only metal object in the house is a cooking pot or a knife. 
Sentence2: sharpening a knife causes it to change shape.
Example Output: knife

Example Input: Sentence1: All plants have a characteristic life cycle that includes alternation of generations. 
Sentence2: Plants have chlorophyll and a characteristic life cycle.
Example Output:
characteristic