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: Bacteria and Viruses Bacteria and viruses can cause diseases. 
Sentence2: bacteria can have a negative impact on an organism.
Example Output: Bacteria

Example Input: Sentence1: Adaptation traits contribute to individual fitness and to the evolution of animal genetic resources. 
Sentence2: Behaviors can improve the evolution of animal genetic resources.
Example Output: animal

Example Input: Sentence1: Birds have a large brain relative to the size of their body. 
Sentence2: BIRD Birds have feathers and wings.
Example Output:
Birds