 So if I'm given a physical quantity with a number and a unit and I don't like that unit I know how to change it into the different number and the other unit that I want just by multiplying by one a few times But why would I prefer one unit over another? There are many reasons to do so for example as a unit of length a rough isn't very good because there's no standard height for a Scottish Terrier so it's very hard to check exactly how many roughs tall you are and same with meows It's very hard to stack cats to very high sort of piles particularly above two or more And so why would I change to different units? Well, sometimes I'd like the numbers to be reasonable I mean I could take a very very tiny unit length like a micron and measure myself in microns But I'll come out to be millions of them and humans don't really like counting around really large numbers You'll notice that when I started talking about three and a half million joules in a previous example I convert it to mega joules because three point six is a kind of human-sized number I can think of three things or four things or three and a half But three and a half million is just a little bit hard for me to conceptualize And so we often change our units to try and make a more reasonable choice of number The absolute easiest way to do this is to use the metric multipliers So when we had three point six million joules, we already introduced the mega mega joule So mega anything is a million of them Or one with six zeros We're familiar with the kilometer being a thousand meters and indeed a kilo anything is a thousand anything's There's also hecto for a hundred and deca for ten and desi for a tenth Probably the next most common one would be the centi a centi something is a hundredth E.g. a cent is a hundredth of a dollar and We have milli a millimetre is a thousandth of a meter and milli is a thousandth of anything We also have micro and nano below that and giga and terra above that and many more besides Once you know those and you know the symbols for these things so giga is Prefixed by a capital G and mega is a capital M and kilo is a little K Micro is the Greek symbol so once you know those symbols you can make up all sorts of units anytime you have a unit You can always have a giga version of that unit or a kilo or a micro So for example, we might have seen Micrometers, so you know what a meter is that's a micrometer. That's a millionth of a meter. You might know kilometers You might be familiar with megahertz a hertz is a unit of frequency It's how many cycles per second so a megahertz is a million cycles per second or in terms of Computers you might have heard of gigahertz. You might have heard of centimeters You might have heard of milliliters. You might have heard of nanometers So many of those examples were meters But in fact all those things could work just as well for jewels You can have mega jewels or micro jewels or you could have mega watts You can have giga watt power stations and so forth