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.

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

Sentence1: Results can be damaging or disastrous. 
Sentence2: Lack of immunization can be damaging or disastrous.
damaging

Sentence1: exercise has a positive impact on a human 's health. 
Sentence2: All movement is exercise.

exercise