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 Input:
Sentence1: Similarly, stars are referred to as suns, but our star is the Sun. 
Sentence2: our star causes water to evaporate more quickly by adding heat.

Ex Output:
our


Ex Input:
Sentence1: All oaks are trees. 
Sentence2: Cutting down oaks has a negative impact on an ecosystem.

Ex Output:
oaks


Ex Input:
Sentence1: a polar bear requires a cold environment. 
Sentence2: polar bears live where there are fewer animals.

Ex Output:

bear