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.

[EX Q]: Sentence1: Cell death occurs by activation of cell death genes. 
Sentence2: Aging is associated with the activation of certain genes.
[EX A]: activation

[EX Q]: Sentence1: gills are used for breathing water by aquatic animals. 
Sentence2: gills are used for breathing water by sharks.
[EX A]: are

[EX Q]: Sentence1: Playing is one way mammals and other animals learn how to behave. 
Sentence2: Playing is really fun.
[EX A]:
Playing