A good question. The kilo-gram was 'invented' during the French revolution. Part of the revolution was to make all things 'new' and this included weights and measures. At this stage the kilo-gram was defined as the mass of a cubic decimetre of water. They then set about finding how much a cubic decimetre of water actually was because it was soon realised that it wouldn't be a particularly good reference standard. In 1786 the kilo-gram was defined as the mass of a cylinder of metal (not sure if it was platinum) and eventually as a cylinder of platinum- iridium alloy. This is called the kilo-gram prototype and is kept in Paris. There is another one in London and recently it has been found that the two now have a very very small difference. That is not good news from a physicist's point of view and there are plans to redefine the kilo-gram in 2011. It will remain the SI unit of mass and be the same mass but it will be defined in terms that cannot change.