There is a lot of black stuff under the ground. Most of it is made from things that lived in big areas of water many years ago. After they died, they fell to the bottom of the water, and other things fell on top of them until they became part of the ground. The ground on top of the dead things pressed on them very hard, and they got very hot. This made the dead things start to turn into black stuff, which took hundreds of hundreds of hundreds of years.
People pay lots of money to get the black stuff out of the ground, because it is very easy to burn, and lots of interesting things (like cars) work by burning things. But before you can burn it, you need to sort it out into different parts. Some parts are thick and heavy, and we use them to make the hard ground on top of roads. Other parts are so light that they look just like air; we used to burn them to light our houses. The parts that people want the most are somewhere in between, because they are easiest to burn in cars.
To sort out the parts of the black stuff, you need to make it very hot–so hot that some of it starts to look like air. Then you let it all cool down very slowly, and you take away each part as it starts to look like water again, which happens to the heaviest parts first and then to the lighter parts. There are also a few ways to turn the heavy parts into light and in-between parts, so that people will pay more money for them. Usually, you make the heavy parts very hot and press them very hard, then run them through a bed of stuff that makes them want to break up into lighter parts. (It helps if you add some of the kind of air that once burned a big sky bag and made someone say “Oh, the [people]!”)
Remember when I said that the black stuff is very easy to burn, and then I talked about making it very hot? Does that make you worried? Well, it makes the people who do it worried too. If something goes wrong, they can set their whole shop on fire, and a lot of people might die. So they have a group of people who go around asking each other what is the worst thing that can happen in any part of the shop.
Let's say that group is looking at a fire box that they use for making black stuff hotter. They might ask what happens if the fire goes out, but they don't notice it and they keep sending stuff into the fire box for burning. If too much stuff-for-burning builds up in one place, then it can burn all at once when the fire comes back on, breaking the fire box and setting fire to everything around it. That's one of the worst things that can happen in a fire box, so they will probably call my shop and ask us how to stop it.
My shop will come up with an idea for a computer that recognizes a bad situation and stops it from getting any worse. In this case, the computer should be able to tell when the fire goes out by feeling that the fire box is getting cold, or by seeing that there is no more light coming out. Then it should shut off all the stuff-for-burning without waiting for the people who run the fire box to tell it to do that. One of the first things to figure out is just how sure we need to be that the computer will save us when we need it. (That helps us decide how many of each part we should use to do the same thing.)
My job is to write down (and sometimes draw) the plan for the computer. Other people usually come up with the big ideas, but there are still a lot of things that need to be decided. What should the computer do if part of it breaks, so that it can't be sure whether something bad is about to happen or not? How does it need to talk to the people running the shop? If something bad almost happens but the computer stops it, when is it okay to go back to normal? I have to come up with enough answers so that there is no doubt about the way the computer should be built.
While the computer is being built, I check out every part of it to make sure everything works well together. Finally, I check out the whole thing by making the computer think the fire is out, then showing the people who are buying it that it really does shut off the stuff-for-burning.
A different group might look at the same fire box and notice that it is almost always burning stuff. Most stuff makes different kinds of bad air when it burns, and the state says you're only allowed to put so much bad air into the sky. My shop also comes up with plans for computers that watch fire boxes for bad air going into the sky and write down just how much they see, to show the state that everything is okay.
Another part of my job is to go into a black-stuff-sorting-out shop and figure out just what kind of computers they have running, and what they are supposed to do. (These kinds of shops are very big, and parts of them are so old that people might not remember everything about the way they built them.) Then I write down everything I find out and help them keep better track of it next time.