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.

Sentence1: Any antigen that causes an allergy is called an allergen. 
Sentence2: Antigens are proteins foreign to the body.
antigen

Sentence1: Silica sand is primarily mined from sandstone deposits. 
Sentence2: erosion forms silica sand.
Silica

Sentence1: Populations of viruses do not grow through cell division because they are not cells. 
Sentence2: populations of HIV do not grow through cell division because they are not cells.

Populations