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: Refraction is the bending of light. 
Sentence2: a rainbow is formed by bending of light by separating light into all different colors.
Example Output: bending

Example Input: Sentence1: a wildfire changes an ecosystem drastically. 
Sentence2: Forest fires change an ecosystem drastically.
Example Output: changes

Example Input: Sentence1: Unlike vertebrates, lancelets lack a hardened backbone. 
Sentence2: some larvae lack a hardened backbone.
Example Output:
backbone