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 air for survival. 
Sentence2: It is breathed in by animals and restored to the air by the photosynthesis mechanism of plants.
Example Output: air

Example Input: Sentence1: Plants that shed their leaves seasonally each year are called deciduous plants. 
Sentence2: vines shed their leaves seasonally each year.
Example Output: each

Example Input: Sentence1: Frozen fossil animals are found in permafrost. 
Sentence2: If frozen fossils are found in permafrost, that plant used to live in that place.
Example Output:
Frozen