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: Sound vibrations travel at different speeds through different materials. 
Sentence2: the type of material through which sound passes changes the sound vibrations.
Example Output: Sound

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: Wind speeds are in knots. 
Sentence2: Anemometers are used to measure knots.
Example Output:
knots