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 requires a source of electricity to produce light. 
Sentence2: Flashlights provide the safest emergency lighting source.

Ex Output:
flashlight


Ex Input:
Sentence1: Parasites are organisms that live off a host. 
Sentence2: roundworms may be free-living or live off a host.

Ex Output:
host


Ex Input:
Sentence1: Oil is one type of fossil fuel. 
Sentence2: oil is formed from dead organisms over time.

Ex Output:

Oil