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

Example Input: Sentence1: an animal requires air for survival. 
Sentence2: It is breathed in by animals and restored to the air by the photosynthesis mechanism of plants.
Example Output: air

Example Input: Sentence1: Neurons are the basic functional unit of the brain. 
Sentence2: the basic functional unit of the brain transmit nerve impulses to other cells.
Example Output:
basic