TASK DEFINITION: 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.
PROBLEM: Sentence1: a fire requires oxygen to burn. 
Sentence2: Oxygen has  eight protons and eight electrons.

SOLUTION: oxygen

PROBLEM: Sentence1: wind is used for producing electricity. 
Sentence2: Winds are caused by pressure differences.

SOLUTION: wind

PROBLEM: Sentence1: Plants are Earth's producers. 
Sentence2: A greenhouse is used to protect producers by keeping them warm.

SOLUTION:
producers