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: drought is when amount of precipitation decreases. 
Sentence2: Drought decreases the availability and abundance of the natural food supply.
[EX A]: drought

[EX Q]: Sentence1: Temperature falls from lower to higher altitudes. 
Sentence2: Temperatures are cooler in the higher altitudes of the mountains.
[EX A]:
altitudes