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.
One example: Sentence1: pesticides cause pollution. 
Sentence2: pollution can harm animals.
Solution is here: pollution.
Explanation: The word "pollution" is common to Sentence1 and Sentence2. So, it's a good answer.

Now, solve this: Sentence1: an animal requires air for survival. 
Sentence2: an animal requires oxygen for survival.
Solution:
animal