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: Some of the best work in ecology has been done on small fresh water pond ecosystems. 
Sentence2: Lakes are generally bigger and deeper than small fresh water ecosystems.

ecosystems