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: Cells are programmed to divide only a set number of times. 
Sentence2: Cells reproduce asexually, by dividing.

Ex Output:
divide


Ex Input:
Sentence1: Placental mammals are therian mammals in which a placenta develops during pregnancy. 
Sentence2: therian mammals give birth to live young, and house the embryo until it is complete.

Ex Output:

mammals