White smoke coming from a car's exhaust can stem from a variety of causes, the most common being condensation. Because of condensation, which occurs naturally as engines cool, water can become trapped in the exhaust pipes. Steam forms as the car heats up, escaping through the tail pipe.
Additional causes of white smoke coming from a tailpipe can be more serious. An internal coolant leak, for example, will produce white smoke. A sweet odor will accompany the white smoke when this is the case. These symptoms will also mean one or more of the engine seals has failed. That can lead to more serious problems such as blown head gaskets or engine warping and result in costly repairs.