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: Consider Input: Sentence1: Populations Populations are subsets of a species. 
Sentence2: Gene flow occurs when individuals move into or out of a subset of a species.

Output: species


Input: Consider Input: Sentence1: Streams may start with runoff or water seeping out of a spring. 
Sentence2: Most water comes from precipitation.

Output: water


Input: Consider Input: Sentence1: Invertebrates are animals who lack a backbone or spinal column. 
Sentence2: Vertebrates are all chordates that have a spinal column.


Output: column