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: Heat production is increased by shivering and non-shivering thermogenesis . 
Sentence2: Heat production occurs when many muscles contract a little bit all at once.
[EX A]: Heat

[EX Q]: Sentence1: Cells are the units of function in living things. 
Sentence2: living things cannot survive without oxygen.
[EX A]: living

[EX Q]: Sentence1: After all, seals are mammals just as humans are mammals. 
Sentence2: Playing is just one of many ways that humans and other animals learn how to behave.
[EX A]:
humans