 So we heard a cautionary tale how it's really important to get the dimensions of a physical quantity, right? If you're talking about energy, it better be a unit of energy. If you're talking about power, it better be a unit of power, and so on. But of course, you also have to actually get the units, right? There's no point specifying something to be 10 meters tall when you actually meant 10 inches tall, and indeed in 1998 the Mars climate orbiter was lost due to a miscommunication in units. They specified thrust in terms of pound seconds instead of Newton seconds. So all I had to do to make that work is to be able to change units. So let's make sure that we can change units. There's a simple trick you can use. So let's take our example of a kilowatt hour again. So a kilowatt times an hour is a unit of energy, and we can turn that into another unit of energy equals one kilowatt hour times one times one. I can always multiply by one. It doesn't change anything. So I can put anything inside here that adds up to one. So what I'm going to put is I'm going to put, I know what a kilowatt is, a kilowatt is a thousand watts. And I've put the kilowatt on the bottom there. So this is one. This is one here because a thousand watts is exactly equal to a kilowatt. That's what a kilowatt means. And then I've put them in that order because that allows me to cancel the kilowatts there. And so now I have a thousand watt hours. But I'm going to go further and I'm going to say, well, I want to change the hours to seconds. So I'm going to say, well, I know what one hour is. I'm going to put the hour on the bottom because I want it to cancel with the hour on the top over there. I know that that's equal to 60 minutes. And so we can say that 60,000, and note that the hours cancel, watt minutes is equal to one kilowatt hour. That's still not a particularly standard unit of energy. So let's keep going. So I've got 60,000 watt minutes. Now, what's a watt? A watt is a joule per second. That's how it's defined. And I've got a minute on the top here. So I want to put a minute on the bottom there. So I'm going to times by one again. I'll put one minute on the bottom. I put 60 seconds on the top. And what I can see is that my minutes cancel, but also my seconds cancel. And I'm left with 60 times 60,000. And that is now in joules. Or if I wanted to make it simpler, I could note that I've got 3.6 megajoules because a megajoule is a million joules. Okay, so that's a way we can just convert. So the trick there is we just keep on multiplying by one. Let's do another example. I heard an American talk about their extremely high prices of petrol recently. So I thought let's try and figure out whether it's cheaper or more expensive than it is in Australia. So I just looked up the price of petrol in the USA. And on average right now, it's $2.23 US dollars per gallon. Okay, now I don't personally know how much a US dollar is worth in an Australian dollar, so I have to look that up. And I don't personally know how big a gallon is, so I have to look that up. And when I do, I can write them in as conversions. And I do it the same way I did it the last time by multiplying by one. So I want the gallon on the top so it cancels with the gallon on the bottom. And one gallon it turns out is worth 3.785 litres. And I also want to convert the US dollars to Australian dollars. And this time I'm going to want the US dollar on the bottom so it cancels with the US dollar on the top. And it turns out that's equal to $1.34 Australian dollars. And now we see we can cancel the US dollars, we can cancel the gallons, and we end up with Australian dollars per litre. And all we've done is we've multiplied by one, because a gallon is exactly equal to 3.785 litres, and we've multiplied by one again, because $1.34 in Australian is equal to a $1 US. And so we end up with 2.23 times 1.34 divided by 3.785. And our final unit is Australian dollars per litre, which is the unit I'm used to seeing on petrol pumps, so I'll be able to understand whether that's a big or a small number. So I plug that into a calculator or do my arithmetic in my head and I get 78.9 cents per litre, which I must say does look like a small number to me.