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: an animal requires warmth for survival. 
Sentence2: Light and heat and warmth are basic necessities for survival.
Example Output: warmth

Example Input: Sentence1: a light bulb is used for seeing in the dark. 
Sentence2: light is required for seeing.
Example Output: seeing

Example Input: Sentence1: Plants use unsaturated fatty acids to store energy. 
Sentence2: Fish oils contain highly unsaturated fatty acids.
Example Output:
acids