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: Mollusks are invertebrates such as the common snail. 
Sentence2: Another common snail is the decollate snail.

A: snail
****
Q: Sentence1: All oaks are trees. 
Sentence2: Cutting down oaks has a negative impact on an ecosystem.

A: oaks
****
Q: Sentence1: Bottleneck effect occurs when a population suddenly gets much smaller. 
Sentence2: Fire usually causes temporary declines in populations.

A:
population
****