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: a sifter is used for separating mixtures. 
Sentence2: a sifter is used for some recipes.

Output: sifter


Input: Consider Input: Sentence1: Habitat destruction is the primary cause of extinction or threat of extinction for all animals. 
Sentence2: building housing developments may cause extinction.

Output: extinction


Input: Consider Input: Sentence1: Mammals have a layer of fat under the skin to help insulate the body. 
Sentence2: Mammals have a layer of fat under the skin to provide warmth and comfort.


Output: Mammals