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: Animals behave in a variety of ways which enhance their chances of survival and reproduction. 
Sentence2: Animals learn to behave to increase chances of survival.
Example Output: behave

Example Input: Sentence1: Many plants respond to the days growing shorter in the fall by going dormant. 
Sentence2: Many plants respond to the autumnal equinox by going dormant.
Example Output: Many

Example Input: Sentence1: competition may cause animals to fight towards members of their own species. 
Sentence2: competition may cause humans to fight each other.
Example Output:
cause