Almost all domestic and wild dogs show this behavior as they prepare to curl up and lie down, but there have been almost no experiments to find out why.
Before giving an answer, let's think for a second about the kind of thing we can learn even from so commonplace an event.
It waddles around, covers itself with mud, and eats plants. Bor-ing. However, hippos are a lot cooler than we normally allow. In fact, hippos never sweat.
Could rearing domesticated zebras raise a profit? Find out on this Moment of Science.