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: Silica sand is primarily mined from sandstone deposits. 
Sentence2: erosion forms silica sand.

Ex Output:
Silica


Ex Input:
Sentence1: Cell death occurs by activation of cell death genes. 
Sentence2: Aging is associated with the activation of certain genes.

Ex Output:
activation


Ex Input:
Sentence1: erosion of soil has a negative impact on the environment. 
Sentence2: cattle can have a negative impact on the environment.

Ex Output:

environment