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: keeping food cold is used for keeping food fresh. 
Sentence2: Keeping food cold is used to keep food simple and natural as possible.

Ex Output:
cold


Ex Input:
Sentence1: lightning can cause a forest fire. 
Sentence2: Lightning can cause a natural cycle in forests.

Ex Output:
lightning


Ex Input:
Sentence1: a light bulb requires electrical energy to produce light. 
Sentence2: a light bulb requires a power plant to produce light.

Ex Output:

bulb