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.

Ex Input:
Sentence1: Speciation events are thus examples of macroevolution. 
Sentence2: Speciation events occur over geologic time above the level of the species.

Ex Output:
Speciation


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

Ex Output:
Mammals


Ex Input:
Sentence1: microorganisms cause food to spoil. 
Sentence2: Tens of millions of Americans get sick every year from bad food.

Ex Output:

food