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: Graphite and diamond are polymorphs of carbon. 
Sentence2: carbon polymorphs are used for making pencil lead.
Example Output: carbon

Example Input: Sentence1: plant reproduction requires pollination. 
Sentence2: Plant reproduction requires insects and hummingbirds.
Example Output: plant

Example Input: Sentence1: Lung cancer is most often caused by exposure to tobacco smoke. 
Sentence2: Many patients with lung cancer also have emphysema.
Example Output:
Lung