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 flashlight converts chemical energy into light energy. 
Sentence2: PALight Flashlights are the ultimate personal LED flashlights.

Ex Output:
flashlight


Ex Input:
Sentence1: Cell death occurs by activation of cell death genes. 
Sentence2: Aging is associated with the activation of certain genes.

Ex Output:
activation


Ex Input:
Sentence1: plasma is formed by electrons separating from atoms in stars. 
Sentence2: electrons separate in stars.

Ex Output:

electrons