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: a flashlight converts chemical energy into light energy. 
Sentence2: PALight Flashlights are the ultimate personal LED flashlights.
Example Output: flashlight

Example Input: Sentence1: the mass of a planet causes the pull of gravity on that planet. 
Sentence2: the mass of a planet is what causes things to have weight.
Example Output: mass

Example Input: Sentence1: Similarly, stars are referred to as suns, but our star is the Sun. 
Sentence2: our star causes water to evaporate more quickly by adding heat.
Example Output:
our