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: erosion of soil has a negative impact on the environment. 
Sentence2: erosion of soil has a negative impact on forests.

Ex Output:
erosion


Ex Input:
Sentence1: Leeches are external parasites. 
Sentence2: Leeches live on the surface of their hosts.

Ex Output:
Leeches


Ex Input:
Sentence1: an animal requires warmth for survival. 
Sentence2: Animals act through their instincts for survival.

Ex Output:

survival