Billy is cruising along the highway when his Motorcycle starts misfiring, losing power and, finally, stalling. Bill steers the bike to the side of the road and tries to re-start the engine without any luck. He calls a bike shop whose service man arrives a short while later, listens to the Bills account of the bikes failure and lifts the tank off and starts work. As he knows nothing about engines, Bill wanders off a little way for a smoke. As he is returning to his bike, the engine suddenly starts and runs without a single misfire or puff of smoke from the exhaust. "Brilliant!" declares Bill, "You managed to fix it. Was it easy to do?". "Yeah", says the service man "Just shit in the carburetor". Billy thinks for a second and says, "OK. How often should I do that then?"