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: keeping food cold is used for keeping food fresh. 
Sentence2: Keeping food cold is used to keep food simple and natural as possible.

Ex Output:
cold


Ex Input:
Sentence1: creating paper requires cutting down trees. 
Sentence2: Trees begin as epiphytes on host trees.

Ex Output:
trees


Ex Input:
Sentence1: threatening behavior causes a fight-or-flight response in animals. 
Sentence2: Goats are four legged animals.

Ex Output:

animals