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.

Ex Input:
Sentence1: Sound vibrations travel at different speeds through different materials. 
Sentence2: the type of material through which sound passes changes the sound vibrations.

Ex Output:
Sound


Ex Input:
Sentence1: Bivalves feed on plankton and nonliving organic matter. 
Sentence2: Clams feed on plankton and nonliving organic matter.

Ex Output:
feed


Ex Input:
Sentence1: Courtship is behavior that is intended to attract a mate. 
Sentence2: Mating Ducks usually look for a mate or partner in winter.

Ex Output:

mate