 Now, for an internal combustion engine, the efficiency is typically only about 25%. So when you burn a litre of petrol to drive somewhere, one litre of petrol turns into actual motion, whereas another three litres of petrol are just heating up your engine. That's why your engine gets so hot that you need fans to cool it down so that the air heat can spread around the environment. Of course, even the energy that goes into motion will end up as heat. As you're driving, you'll be stirring up the air around it. You'll be bending your tyres, and all that will end up as heat as well. So actually 100% will end up as heat eventually, but 25% of it is useful and makes the car go. For a big power station, it's more like 50%. A power station, a good one, can be around 50% maybe, usually a bit worse than that, but occasionally slightly better. So about half the energy is wasted turning into heat. That's why power stations need things like these cooling towers to dissipate the other 50% of energy that's going out in the form of heat. It's a real waste and it's led to the idea of combined heat and power. The idea here is instead of having a whopping great power station out in the middle of nowhere, where half the energy gets turned into electricity and half is wasted on heating up the air and the sea and the water around it, maybe you have lots of small power stations actually building people's houses or neighbourhoods. And so all the heat that's wasted could be pumped down a vent or something into people's houses and keep them warm. And if you're in a cold country, like England or Sweden or Canada, where you need a lot of heat, that can make it much more efficient. They'll be twice as good as the conventional power stations. Probably not so useful in a hot country like, say, Queensland on a hot summer's day when you want to electricity to run the air conditioning. Oh well. So that's efficiency and it's a fundamental thing that engineers have to worry about.