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 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: Capillaries are microscopic vessels. 
Sentence2: Lymph is a fluid that leaks out of microscopic vessels into spaces between cells.

Ex Output:
microscopic


Ex Input:
Sentence1: Sound waves are vibrations in matter. 
Sentence2: a rubber band snapping causes the particles in the rubber band to cause sound waves.

Ex Output:

Sound