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: Sperm had to swim from male to female reproductive organs for fertilization. 
Sentence2: If fertilized, the egg implants itself in the uterus.

Ex Output:
fertilization


Ex Input:
Sentence1: Interstellar space contains a plasma. 
Sentence2: interstellar space contains electrons, atoms, and stars.

Ex Output:
Interstellar


Ex Input:
Sentence1: Running water is better able to dissolve oxygen and nutrients than standing water. 
Sentence2: Streams and creeks provide running water.

Ex Output:

Running