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: Caverns Secrets are hidden in caves. 
Sentence2: hidden caves are formed by carbonic acid in groundwater seeping through rock and dissolving limestone.

Ex Output:
are


Ex Input:
Sentence1: Earth's Atmosphere Earth's atmosphere is the layer of gasses that surrounds the earth. 
Sentence2: weather refers to the conditions in the layer of gasses that surrounds the earth.

Ex Output:
earth


Ex Input:
Sentence1: fluoride decreases tooth decay. 
Sentence2: Tooth decay is a destruction of the tooth enamel.

Ex Output:

decay