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: Smoking accelerates the aging process. 
Sentence2: smoking accelerates the death of cells.

Ex Output:
Smoking


Ex Input:
Sentence1: Plants exhibit phototropism, or growing toward a light source. 
Sentence2: Bamboos are vigorous, rugged plants.

Ex Output:
Plants


Ex Input:
Sentence1: clouds are formed by water vapor rising into colder regions of the atmosphere and condensing. 
Sentence2: water vapor sometimes falls as rain.

Ex Output:

vapor