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.

Ex Input:
Sentence1: Graphite and diamond are polymorphs of carbon. 
Sentence2: carbon polymorphs are used for making pencil lead.

Ex Output:
carbon


Ex Input:
Sentence1: Visible and invisible light is energy in the form of electromagnetic waves. 
Sentence2: Eyes are used for seeing animals by sensing electromagnetic waves.

Ex Output:
electromagnetic


Ex Input:
Sentence1: Tunicates reproduce sexually. 
Sentence2: bulbs reproduce sexually.

Ex Output:

reproduce