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.

Input: Consider Input: Sentence1: Heterotrophs are living things that cannot make their own food. 
Sentence2: food contains organic molecules.

Output: food


Input: Consider Input: Sentence1: Mammals generate heat mainly by keeping their metabolic rate high. 
Sentence2: food keeps mammals' metabolic rates high.

Output: Mammals


Input: Consider Input: Sentence1: Natural gas is the cleanest of the fossil fuels. 
Sentence2: Heat and pressure create fossil fuels out of dead prehistoric beings.


Output: fossil