Bison, in Yellowstone National Park, Wyoming. There are constant reminders for tourists to keep their distance from bison, as they are unpredictable. They can weigh 2,000-2,700 pounds and sprint at 30 mph! While they may seem lazy and slow, and even let an occasional stupid tourist approach them extremely closely, you never know when their tolerance will run-out and they'll charge. Part of why I love this photo however, is that it almost seems like an intimate social moment between these two. They were play-fighting, like puppies. You can even see it in the eye of the bison on the left... wanting to play.