The commercial pilot, his passenger, and his 80 to 100 lb dog, were on a cross country flight and had just leveled off at 2,500 ft when the engine suddenly stopped producing power. The pilot was unable to re-start the engine and made a forced landing to a field. During the landing the airplane struck a tree and a fence resulting in substantial damage to the firewall, fuselage, and wings. A postaccident examination of the airplane revealed the right fuel tank had 15 gallons of fuel in it and the left tank was empty. The fuel selector valve was found between the "left" and "both" tank positions. The front right seat had been removed prior to the flight to accommodate the pilot's dog, who sat on the floor during the flight. The pilot said he departed with the fuel selector valve in the "both" position but during the flight the dog must have inadvertently moved the valve's handle toward the "left" tank position, which resulted in the loss of engine power due to fuel exhaustion. A functional check of the fuel selector valve revealed no mechanical issues and the valve seated securely into each detent. The pilot reported there were no mechanical deficiencies that would have precluded normal operation of the engine at the time of the accident.