The measurement of a kilogram is currently based on a hunk of metal kept under lock and key in France - but that's about to change.
This Way Up finds out why.
The kilogram is the last unit of measurement (as defined by the International System of Units) to still be based on a physical object.
From next year. The International Committee for Weights and Measures is redefining the standard kg unit.
No more physical objects - a kilo will be calculated by reference to a physical constant known as Planck's constant or 'h'.
Other units of measurement like the second, the metre, the ampere, the kelvin, the mole and the candela are all based on a physical constant - roughly speaking something that happens in nature and stays the same over time, whether you're in Timbuktu or Timaru.