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: Amphibians are the first true tetrapods, or vertebrates with four limbs. 
Sentence2: Frogs are classified as amphibians.

Ex Output:
Amphibians


Ex Input:
Sentence1: creating paper requires cutting down trees. 
Sentence2: Rainforests are destroyed by logging and all of the trees are cut down.

Ex Output:
cutting


Ex Input:
Sentence1: Mutualism is a symbiotic relationship in which both species benefit. 
Sentence2: oxpecker birds help other species benefit.

Ex Output:

benefit