 So we've seen that heat flows from hot to cold, but how does it get there? Well, there are three different ways in which heat energy can go from a hot thing to a cold thing. Conduction, convection and radiation, and we'll talk about them in turn. Now conduction is perhaps the simplest. Let's say you put a spoon into boiling water and you're holding on to the end up here. You'll feel pretty hot after a short while. The heat is somehow making its way up the spoon towards your fingers. Now how does that do that? Well let's consider some lattice of atoms. They're all connected by chemical bonds, as we've talked about many times. And let's say the send is exposed to heat, like it's put in boiling water. That means the atoms here will be vibrating like crazy, moving all over the place. But all the rest are just going at their slow, small vibrations. You've got to imagine if the atoms here are moving backwards and forwards in enormous speed, because of their chemical bonds it's going to be tugging on this one. And before long that one will start moving. And then this one will in turn tug on this one. And so before long the whole thing will be jiggling. So that's the basic idea behind conduction. That when things are moving really fast, they pull on the chemical bonds and yank around the ones next to them and stop the ones next to them jiggling around. And they in turn keep pulling all the way along. It turns out for metal there's an even more fast effect. That in the lattice of say copper or silver or steel or whatever you've got your spoon made out of, because it's a metal, the electrons, some of them, are free to wander around as they like. They're not bound to the atoms. What happens is when you get the atoms at one end moving like trees, they actually collide with the electrons and the electrons go whizzing down here and they will hit an atom maybe 100 or 200 atoms further away and get that vibrating. And so what's actually happening is like a ping-pong game. The hot nuclei at one end are smashing the electrons like the ping-pong balls which fly down through the metal and hit other atoms much further away and get them moving faster too. That's why metal is such a good heat conductor. It's because the electrons get knocked flying by the fast moving atoms at one end and communicate the energy to another end. So that's conduction. How do you stop it working? Often you don't want too much conduction. For example, in your clothes, if it's a cold day, you want your clothes not to conduct your body heat out into the outside world. Well, the best way to do that is to break up your substance. So instead of having solid atoms from the inside to the outside of your clothes, you might, for example, make it out of fabric, which will have fiber here, fiber here, or woven together in some sort of pattern. And the idea is here, let's say this fiber here is close to your body and therefore the atoms here are jiggling around like crazy and it's very warm. But because there's an air gap between this fiber and the next fiber over there, at a microscopic level, the vibrations here can't affect the vibrations over there. It might be affected when they touch each other, but that's only a small fraction of the area. Instead, the fast moving atoms over here will have to hit the air atoms and stop moving them faster. And then eventually, once the air in this little cavity inside your clothes is warmed up, that will then start hitting the molecules here and get them going faster. And so on. So having these gaps means that this whole effect of one bond pulling on the next can't work. It makes it much slower by forcing the atoms here to interact with the gas and the gas to interact with the atoms and the atoms to interact with the gas, and so on, which is why you wear woolly jumpers and why you don't generally wear sheet metal. Big, pretty cold. Unless, of course, you're a medieval knight. This is also how roof insulation works. You try to get gaps in stuff, so it's generally very fluffy. Even for something like wood, wood is a good insulator usually because it's got tiny fibers of celluloid which have gaps in between them. So the fast moving in one place atoms can't affect the other ones nearby, except by heating up the gas and the spaces inside the wood. Whereas something that's solid, like a stone, has a much better conductor and metal is the best conductor of all.