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: wind is used for producing electricity. 
Sentence2: Wind is caused because of pressure differences in the atmosphere.

Ex Output:
wind


Ex Input:
Sentence1: Stress is the applied force, strain is the change in shape. 
Sentence2: Paper is crumbled when stress and strain is applied in order to change the shape.

Ex Output:
Stress


Ex Input:
Sentence1: Differentiation is the process by which unspecialized cells become specialized. 
Sentence2: Differentiation is the process by which neurones are created.

Ex Output:

Differentiation