Petrol, as we all know only too well these days, is a hydrocarbon carbon fuel so when you burn it in an oxygen rich gas - air - one of the byproducts is H2O - water. I remember being amazed when my college instructor told me that for every gallon of petrol burned slightly more than a gallon of water comes out the exhaust pipe! Of course mostly you don't notice it because, once the engine and exhaust system is hot it comes out the end as superheated steam, which is invisible. In colder weather it will probably condense in the cold air so you see the white smoke and when the engine is first started, so the exhaust is cold, it will condense inside the exhaust and you'll often see, on cold winter mornings, the car in front of you at the lights or in a traffic hold up, dribbling liquid water from the exhaust pipe.
One of the classic signs of a blown head gasket, or anything else which might be letting water into the cylinders, is white exhaust gasses (steam) but, especially with modern cars with Catalytic converters, you'll often see white smoke or water at the end of the exhaust shortly after start up or when idling and people think there's a problem when there isn't. So mj2k's advice above, especially keeping a regular check on coolant level, might be a good way to carry on?
