Definition: 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: Sentence1: Amphibians are the first true tetrapods, or vertebrates with four limbs. 
Sentence2: amphibians are the first true animals with backbones with four limbs.

four