How much is a kilogram? It's a simple enough question at first glance, but the answer is a bit more complicated than it seems.  In this video, Derek Muller from Veritasium talks about how the kilogram is defined and the current efforts to redefine it.

The kilogram, like most other units, is arbitrary; it has no special property that makes it what it is, nor is it not some fundamental feature of nature. Basically, one day somebody declared the mass of a certain object would be equal to one kilogram (originally the mass of one liter of water just above freezing), and we've run with that sort of definition ever since. Starting in 1889 and to this very day, one kilogram is defined literally as the mass of a single golf-ball sized cylinder of platinum-iridium stored in a sealed vault in France. That means this little chunk of metal is the only object in the world of mass exactly equal to one kilogram, because it is the kilogram. The big papa kilogram, if you may.

The problem with this definition is that the masses of objects change; wear, radioactive decay, storage conditions, etc. can all contribute to deviations. Despite our best preservation efforts, this too is true of the official kilogram. The standard suggested in the video would specify the kilogram in terms of the mass of a precise number of silicon-28 atoms, such that we wold not need worry about variances in a singular physical object again. The details are a bit more technical, so check out Veritasium's excellent explanation.