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.

Example Input: Sentence1: Osteoporosis is a disease in which osteoclasts are more active than osteoblasts. 
Sentence2: a thinning and weakening of bone is a disease in which osteoclasts are more active than osteoblasts.
Example Output: active

Example Input: Sentence1: Muscles can only contract. 
Sentence2: Contraction is a type of movement.
Example Output: contract

Example Input: Sentence1: blowing air into a whistle causes air to vibrate inside the whistle. 
Sentence2: blowing air into a whistle creates sounds.
Example Output:
air