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 accounting period is simply a period of time. 
Sentence2: seconds are used to measure a period.
Example Output: period

Example Input: Sentence1: Some viruses have an envelope of phospholipids and proteins. 
Sentence2: Some viruses make their own membranes.
Example Output:
Some