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: 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: Leeches are external parasites. 
Sentence2: Leeches live on the surface of their hosts.
Example Output: Leeches

Example Input: Sentence1: Animals with backbones are called vertebrates. 
Sentence2: hagfish have a backbone.
Example Output:
backbones