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: Allergens can trigger asthma attacks.
allergen

Sentence1: friction causes the temperature of an object to increase. 
Sentence2: Friction causes the temperature of a toy to increase.
causes

Sentence1: a firecracker converts chemical energy into sound. 
Sentence2: Sound waves vibrate.

sound