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: Dilation occurs when cell walls relax. 
Sentence2: Decreased oxygen levels cause blood vessels to dilate and cell walls to relax.
Example Output: relax

Example Input: Sentence1: an image in a mirror is formed by reflecting light. 
Sentence2: mirrors can be seen with our eyes.
Example Output: mirror

Example Input: Sentence1: an animal usually requires a warm body temperature for survival. 
Sentence2: Many warm-blooded animals control their body temperature by sweating or panting.
Example Output:
animal