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: a fire requires oxygen to burn. 
Sentence2: Oxygen has  eight protons and eight electrons.

A: oxygen
****
Q: Sentence1: a plant requires sunlight to grow. 
Sentence2: Grass needs air and sunlight.

A: sunlight
****
Q: Sentence1: Clouds are made of moisture and the moisture is from the water evaporating. 
Sentence2: Lack of water in the air causes low amounts of rainfall.

A:
water
****