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: Eggs are usually laid in a nest. 
Sentence2: birds have nests.

Output: nest


Input: Consider Input: Sentence1: Fertilization occurs naturally in the fallopian tubes. 
Sentence2: fertilization is used for reproduction.

Output: Fertilization


Input: Consider Input: Sentence1: Male gametes are sperm. 
Sentence2: fertilization occurs when male gametes swim to an egg inside an archegonium.


Output: Male