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.

[Q]: Sentence1: cold fronts cause thunderstorms as they pass by. 
Sentence2: Cold fronts cause lightning.
[A]: cold


[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.
[A]: species


[Q]: Sentence1: if a planet becomes too hot then that planet cannot sustain life. 
Sentence2: Sustainable, organic agriculture is predicated on such balanced ecosystems.
[A]:
sustain