Let's say you have this dx right over here, and you rotate it around the x-axis. So if you were to rotate this thing around the x-axis, so I'm trying my best to, around the x-axis, you rotate it, what do you end up with? What do you end up with? Well, you get something that looks kind of like a coin, like a disk, like a quarter of some kind. Let me draw it out here. So if you were to, that same disk out here would look something like this. It would look something like this, and it has a depth of dx.