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: Distance is measured in meters the SI unit of distance. 
Sentence2: The SI unit of distance is in meters.

A: SI
****
Q: Sentence1: Humans have collected and grown mushrooms for food for thousands of years. 
Sentence2: Humans have collected edible mushrooms for thousands of years.

A:
Humans
****