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: cold fronts cause thunderstorms as they pass by. 
Sentence2: Cold fronts cause lightning.

Ex Output:
cold


Ex Input:
Sentence1: plant reproduction requires pollination. 
Sentence2: Plant reproduction requires insects and hummingbirds.

Ex Output:
plant


Ex Input:
Sentence1: Pulped wood is used to make paper. 
Sentence2: pulped wood requires cutting down trees.

Ex Output:

Pulped