Every time we go to the dog park, I see Luna, my black and white whippet, sniffing the ground obsessively. She would then stop, bite the grass, and roll around in that spot for about 2 minutes straight. When I noticed that my other 2 dogs would join her in this ritual, I finally went over to see what the heck they were all rolling in. Turns out they were hunting down earthworms and rolling around on top of them until the earthworms were crushed to a pulp. Gross.