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: Some pelycosaurs gave rise to a group of animals called therapsids. 
Sentence2: Like most other groups of synapsids, all pelycosaurs are now extinct.
Example Output: pelycosaurs

Example Input: Sentence1: a fire requires oxygen to burn. 
Sentence2: If the oxygen is removed, the fire is smothered.
Example Output: oxygen

Example Input: Sentence1: Life depends to a large degree on the temperature of the planet, and temperature depends in part on the distance of the planet from the sun. 
Sentence2: Life cannot be sustained above a certain temperature.
Example Output:
temperature