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: an animal requires air for survival. 
Sentence2: The Moon has no atmosphere and no air.

A: air
****
Q: Sentence1: Lions live in social groups called prides. 
Sentence2: Chimpanzees live in social groups called communities.

A: groups
****
Q: Sentence1: Some arthropods have special excretory structures. 
Sentence2: Insects have special excretory structures.

A:
excretory
****