Remember, we drew a picture of an iron bar. And it looked like the following. We have some set of domains. And in each domain the dipoles aligned in one direction. But from one domain to the next, it's random orientation. In iron, it turns out the electrons have a spin which talks to the neighboring electrons. And so in this domain, that is a region where all those electrons are talking to their neighboring electrons and saying, "let's line up in this direction." And that works for a while until you get to the edge of the domain. And then there's another group down here that does the same thing. What defines those groups? It turns out that the domains arise by minimizing the overall energy configuration. So when you have spins, when you have dipoles that are aligned or not aligned there is some particular energy associated with that configuration. Iron bars, like anything else in their natural state, want to minimize that energy configuration. Okay? You want to minimize your energy configuration because you're on the ground. If you were up in the air, you would have a lot of potential energy. And that's not a minimum. So being on the ground minimizes your energy state. Just like when you knock something off, it falls to the ground, right? But in the bar, there is energy associated with these dipole states and how they talk to each other. And it turns out that the minimum energy state defines these domain walls. Now, to really understand all that you have to get into much more complicated physics. You have to go take solid state physics to really understand it. But what it does tell you is when all these things are aligned. Like we put it in a magnetic field and align all of them. This is no longer the minimum energy state. This is some higher energy state and it doesn't like to be in there. And so how do you get it back to this temperature? I mean to this disordered state? Well, if you raise the temperature close to what's called the Curie temperature And this is named after Marie Curie, okay. Double Nobel laureate, Marie Curie, in physics and chemistry. If you raise it close to that temperature, then the domains get scrambled. Because you have jostled the system and now it's going to fall back to its minimum energy state. And this is something that you might be aware of. I was certainly aware of it when I was a teenager. Because when you take your tape and you leave it in the car and you have all the windows rolled up, it of course gets really really hot in your car. It doesn't get to the Curie temperature, but it starts to get a lot hotter in your car. And if you do that over and over and over again, and you go to play your tape, right? And back in high school, the big popular band for me was Devo, right. Anybody know Devo? They kind of made a second showing recently. But anyway, so Devo, right, the song "Whip It." Whip it good? No? Okay, never mind. Anyway, you play the tape cassette after it's sat in your hot car for a long time and it doesn't sound good anymore. It starts to sound horrible. Okay, and that's because all the information that used to be on your little magnetic strip right there got really hot and got scrambled and went back to something like this. If it really hits the Curie temperature then, it completely destroys the information. But if it's just getting hotter, it will start to scramble the information. This is good information, right? Because this tells you if you have some information on your disk and you want to destroy it. On your tape, and you want to destroy it, you can just heat the whole thing up. That'll do it. Now you guys don't have tape cassettes anymore, but you of course have something in your computer. Which has a lot of information on it. What's that? It's your hard drive, okay. Here's the hard drive. It's spinning around. And on your hard drive is magnetic material. And you take a solenoid, right here. And you write some information onto the hard drive. And this thing spins around and this right head goes in and out. And you can tell this dot to be pointing to the right and this dot to be pointing to the left. And now you can put a whole bunch of information on the hard drive. It's the exact same idea as a tape cassette. It's very, very, not exactly the same, but it's very similar to the tape cassette.