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

Sentence1: Cells are the units of function in living things. 
Sentence2: living things cannot survive without oxygen.
living

Sentence1: Halos are produced by the refraction of light. 
Sentence2: glass can cause halos.

Halos