Imagine two lab rats in their cages – one fat, one thin. The larger rat pads around slowly or rests on the floor of wood shavings. She expends as little effort as possible to reach her water spigot, maybe even lying on her back and gripping it with her little pink paws so that the water drips into her mouth.
Her thin neighbor, on the other hand, darts around the cage, whiskers twitching, eyes alert. Taken out and placed on a treadmill, she picks up the pace and overtakes the top of the belt.