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.

Example Input: Sentence1: hiding can be used for avoiding predators. 
Sentence2: Camouflage is used for avoiding predators.
Example Output: avoiding

Example Input: Sentence1: Fertilization is the penetration of the egg by a sperm. 
Sentence2: Sperm must swim from the male to the female egg.
Example Output: egg

Example Input: Sentence1: Bats are mammals that fly. 
Sentence2: Bats have Hair which is a fiber.
Example Output:
Bats