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: healing requires rest. 
Sentence2: healing requires time.
Example Output: healing

Example Input: Sentence1: Fertilization is the penetration of the egg by a sperm. 
Sentence2: Sperm must swim from the male to the female egg.
Example Output: egg

Example Input: Sentence1: Plant growth is dependent on photosynthesis. 
Sentence2: Darkness has a negative impact on plant growth.
Example Output:
Plant