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.

[Q]: Sentence1: Any antigen that causes an allergy is called an allergen. 
Sentence2: Allergens can trigger asthma attacks.
[A]: allergen


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


[Q]: Sentence1: Reflectance is the ratio of light out to light in. 
Sentence2: An image in a mirror is formed by the ratio of light out to light in.
[A]:
light