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, what can teach animals how to behave?
Choices:
(1). adding heat
(2). humans
(3). fossil fuels
(4). A computer
(5). nerves
(6). people
(7). irradiation
(8). exercise
(8).