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: Chordates have a complete digestive system and a closed circulatory system. 
Sentence2: All chordates are vertebrates.
Example Output: Chordates

Example Input: Sentence1: a fire requires oxygen to burn. 
Sentence2: A fire requires something with exactly eight protons to burn.
Example Output: burn

Example Input: Sentence1: Most modern gymnosperms are trees with woody trunks. 
Sentence2: most modern evergreens are trees with woody trunks.
Example Output:
Most