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: 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: Crops are plants that are grown for food or other uses. 
Sentence2: insects destroy plants.

Ex Output:
plants


Ex Input:
Sentence1: a light bulb requires electrical energy to produce light. 
Sentence2: Bulbs require electrical energy to see in darkness.

Ex Output:

bulb