Frank Tippett was injured while snowmobiling in Yellowstone National Park. A moose had charged another snowmobile earlier in the day, knocking two passengers to the ground. Dave Phillips, a Yellowstone park ranger, learned of the moose and monitored its activities throughout the day. When Tippett approached the moose in his snowmobile, Phillips directed Tippett to pass the moose. As Tippett passed the moose, it charged Tippett’s snowmobile, knocking him to the ground and injuring him. Tippett and his wife (plaintiffs) filed claims for negligence against the United States (defendant) under the Federal Tort Claims Act (FTCA). The district court dismissed the claims as barred by the discretionary function exception to the FTCA. Tippett and his wife appealed.