Think about it, if someone asks you what a kilogram is, that’s easy, right? You say it’s a unit of measuring weight (or you may even say of measuring mass if you’re really picky). But then they say how much does a kilogram weigh, how is it defined, and it becomes a little trickier. We could change it around and say one kilogram is the same as 2.2 pounds, although that doesn’t really help matters.
There are seven basic units of measure in Physics, distance measured in metres (m), time measured in seconds (s), temperatures measured in kelvins (K) and mass measured in kilograms (kg) are the most well-used. The additional unit of measure are electric current measured in amperes (A), the amount of substance in mols (m) and the brightness of light measured in candela (cd).
If I had asked you to define any of the other units of measure then you’d probably have to think, but a vague guess at the answer would be easier. For example, a metre is defined by the speed of light, with one metre being the distance travelled by light, in a vacuum, in 1/299792458 seconds. Which leads to the equally difficult how-do-you-define-a-second problem. Well, I won’t get into the specifics here, but one second is the length of time it takes for a microscopic fluctuation in a Caesium atom!
So the point I’m trying to make is that there are physical references for these units, you can relate them to someone else by telling them the experimental method and then they can work it out for themselves. Or perhaps that should be “could”, since I can’t imagine anyone these days needing to verify a second, the science buffs have made so many excellent measuring instruments over the years it seems unnecessary.
Which brings me back to the kilogram. It’s not like this, if you wanted to get someone to find out for themselves then they would have to go and ask very very nicely if the French people that look after it would let you use it. Although I have to admit they’d probably say no, since it’s quite an important thing. Every year they have to measure it carefully and see if it’s lost any weight through atoms escaping the surface, so someone getting their grubby hands on it is probably unlikely. I’ve never seen it, but apparently it’s a small cylinder of platinum-iridium about the size of a golf ball. I still prefer to imagine it like this: