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 flashlight converts chemical energy into light energy. 
Sentence2: Chemical energy converted to light energy requires batteries.
chemical

Sentence1: friction causes an object to lose energy. 
Sentence2: Friction occurs when surfaces rub together.
friction

Sentence1: Iron is involved in the formation of chlorophyll. 
Sentence2: Iron is required for plants to absorb light energy.

Iron