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

Example Input: Sentence1: erosion causes a river to become deeper and wider. 
Sentence2: On the contrary, the NILE is the longest river in Africa.
Example Output: river

Example Input: Sentence1: Fertilization occurs when sperm swim to an egg inside an archegonium. 
Sentence2: fertilization occurs when sperm swim inside a single egg embedded in the prothallus.
Example Output:
Fertilization