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: 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: moss causes chemical weathering. 
Sentence2: If the moss was completely dried then it is dead.
Example Output: moss

Example Input: Sentence1: creating paper requires cutting down trees. 
Sentence2: Creating paper destroys rainforests.
Example Output:
creating