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: a seismometer is used to measure the strength or magnitude of an earthquake. 
Sentence2: Seismometer Calibration : Instructions on how to calibrate a homemade seismometer.

Ex Output:
seismometer


Ex Input:
Sentence1: a rainbow is formed by refraction of light by separating light into all different colors. 
Sentence2: Rainbows form when raindrops refract sunlight.

Ex Output:
refraction


Ex Input:
Sentence1: Telescopes are the tools of the astronomers to study the unverse. 
Sentence2: Astronomers study the stars and universe.

Ex Output:

unverse