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: looking at bright objects has a negative impact on the eyes. 
Sentence2: Looking at bright objects have a negative impact on the retina.

Ex Output:
bright


Ex Input:
Sentence1: An accounting period is simply a period of time. 
Sentence2: seconds are used to measure a period.

Ex Output:
period


Ex Input:
Sentence1: Ethanol is produced by alcoholic fermentation of the glucose in corn or other plants. 
Sentence2: fermentation can be used to make fuel.

Ex Output:

fermentation