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: Fish have a two-chambered heart while mammals and birds have four chambered hearts. 
Sentence2: Mammals and fish have different circulatory systems.

mammals