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: Antigens are proteins foreign to the body. 
Sentence2: antigens are proteins.
Example Output: proteins

Example Input: Sentence1: After a seed is planted it is watered. 
Sentence2: seeds need water.
Example Output: seed

Example Input: Sentence1: Plants exhibit phototropism, or growing toward a light source. 
Sentence2: First, the sun is a brilliant source of white light.
Example Output:
light