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 Q]: Sentence1: Populations Populations are subsets of a species. 
Sentence2: Gene flow occurs when individuals move into or out of a subset of a species.
[EX A]: species

[EX Q]: Sentence1: skin is used for protecting the body from harmful substances. 
Sentence2: Acids are harmful to tissue.
[EX A]: harmful

[EX Q]: Sentence1: For example, hearing waves are transmitted by fluid in the inner ear. 
Sentence2: fluid in the inner ear is used for sensing sound by animals.
[EX A]:
ear