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: a fire requires oxygen to burn. 
Sentence2: A fire requires something with exactly eight protons to burn.

Ex Output:
burn


Ex Input:
Sentence1: Erosion of rock makes nutrients available. 
Sentence2: rain causes nutrients to be available.

Ex Output:
nutrients


Ex Input:
Sentence1: a freezer is used for cooling things. 
Sentence2: An ocean breeze and a freezer keep things cold.

Ex Output:

freezer