For every gallon of gasoline you burn (@14.7:1 A/F ratio) you produce 1 gallon of water. plan and simple. Keep am eye on it. It'll go away after it warms up. When the exhaust system is cold, the water vapor (produced during the combustion process, see above) turns to actual water, (from steam) and condensates on the pipe walls. Then enough water droplets form and then get blown out the tail pipe. It will stay in a vaporous form once the car heats up and all the pipes are up to temp, and the steam will go away. Ever notice cars steaming on cold days in traffic? That's because it takes longer for the tubing to reach tempurature, allowing the vapor to stay in a vaporous state.