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: boiling means change from a liquid into a gas by adding heat energy. 
Sentence2: boiling changes a liquid into steam.
Example Output: liquid

Example Input: Sentence1: Ice is frozen water. 
Sentence2: ice expands.
Example Output: Ice

Example Input: Sentence1: Mammals breathe with the help of a diaphragm. 
Sentence2: Belly breathing uses the diaphragm, which helps relax the lungs.
Example Output:
diaphragm