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: when a gas in an open container evaporates , that gas spreads out into the air. 
Sentence2: when a gas in an open container evaporates gas molecules slamming into one another.
Example Output: container

Example Input: Sentence1: the Earth being tilted on its rotating axis causes seasons. 
Sentence2: the planet's orbit and tilt causes seasons.
Example Output: seasons

Example Input: Sentence1: lightning can cause a forest fire. 
Sentence2: Lightning bolts are cascades of electrons.
Example Output:
lightning