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: healing requires rest. 
Sentence2: healing requires time.
Example Output: healing

Example Input: Sentence1: Sound waves are vibrations in matter. 
Sentence2: a firecracker converts chemical energy into vibrations in matter.
Example Output:
matter