Fact 1: Playing is one way mammals and other animals learn how to behave.

Fact 2: Exercise is fun if it is also play.

Given the two facts above, answer the question "What can teach animals how to behave?" with the following options: 
- adding heat
 - humans
 - fossil fuels
 - A computer
 - nerves
 - people
 - irradiation
 - exercise


 

        exercise 