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: friction causes an object to lose energy. 
Sentence2: Friction occurs when surfaces rub together.

Ex Output:
friction


Ex Input:
Sentence1: erosion causes a river to become deeper and wider. 
Sentence2: On the contrary, the NILE is the longest river in Africa.

Ex Output:
river


Ex Input:
Sentence1: Wind can cause damage to buildings and bring down power lines. 
Sentence2: uneven heating of the earth's surface can cause damage to buildings.

Ex Output:

buildings