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: a fire requires oxygen to burn. 
Sentence2: A fire requires something with exactly eight protons to burn.
burn

Sentence1: a flashlight requires a source of electricity to produce light. 
Sentence2: Flashlights provide the safest emergency lighting source.
flashlight

Sentence1: Proteins are an essential nutrient for heterotrophs . 
Sentence2: Proteins are an essential nutrient for animals.

Proteins