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: air pollution can cause the pH of soil to decrease. 
Sentence2: Acidity is measured with a pH meter or with pH paper strips.
Example Output: pH

Example Input: Sentence1: Bone matrix consists of collagen and minerals. 
Sentence2: collagen provides protection for soft tissues and internal organs.
Example Output:
collagen