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: a polar bear requires a cold environment. 
Sentence2: polar bears live where there are fewer animals.
[A]: bear


[Q]: Sentence1: Macroevolution occurs over geologic time above the level of the species. 
Sentence2: adding new genes with special capability occurs over geologic time above the level of the species.
[A]: above


[Q]: Sentence1: a rainbow is formed by refraction of light by separating light into all different colors. 
Sentence2: Refraction is the bending of light.
[A]:
refraction