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: Acid rain is precipitation which pH is 5.7 or lower. 
Sentence2: gases from burning coal causes precipitation which pH is 5.7 or lower.

Ex Output:
5


Ex Input:
Sentence1: Clouds are one form of condensation. 
Sentence2: Clouds are one form of changing from a gas into a liquid by reducing heat energy.

Ex Output:
Clouds


Ex Input:
Sentence1: water causes refraction of light. 
Sentence2: When light shines through a drop of water or prism, it bends or is refracted inside the droplet.

Ex Output:

refraction