Have you been reading one of those lists of "stuff you didn�t know" on the internet, sonic? Most of that stuff is b0ll0cks, and stuff you didn�t want to know in the first place. A kilogram of hot water is definitely heavier than 100 grams of cold water (stating the obvious), but hot water is generally less dense than cold water except as stated above. The explanation for that, by the way, is that the crystal structure of ice is quite "open". Shortly before it freezes, water starts to take on this "open" structure, even though it is still completely liquid (as dundurn said). Please see my earier question "Why is there so much rubbish on the internet?" which I started with exactly your question about water. Another thing, when other posters have mentioned the mass of the energy added to the hot water, we are talking about incredibly small, almost undetectable, amounts of mass. It�s the Einstein equation, E=mc^2.