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: Skin color is another polygenic trait. 
Sentence2: Skin color is influenced by multiple genes.
Example Output: Skin

Example Input: Sentence1: Plants exhibit phototropism, or growing toward a light source. 
Sentence2: Phototropism means growing toward the energy of light.
Example Output: phototropism

Example Input: Sentence1: a flower 's purpose is to produce seeds. 
Sentence2: a flower 's purpose is to produce for propagation.
Example Output:
flower