Question

Define one kilogram

Answer 1

1 kilogram was originally defined as the mass equivalent to 1000 cubic centimeters of water at around 4 degrees C (about 40 °F). (1 gram was the amount in 1 cubic cm.) But this wasn't very useful, so a standard mass in the shape of a cylinder was manufactured using an alloy of platinum and iridium, as close to the defined mass as possible. This standard kilogram is now used to DEFINE the kilogram, and is kept safely in storage near Paris. Some day maybe the definition will be upgraded (just as the definition of the second was changed a few times to improve the accuracy).