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: molds are antigens.
antigen

Sentence1: an earthquake causes the ground to shake. 
Sentence2: A number of buildings in Istanbul also were damaged in the earthquake.
earthquake

Sentence1: a calculator is used for adding. 
Sentence2: From our Calculators page, click on Financial Calculator.

calculator