That's this blue area right over here. And then if I want to find the volume of this coin, I would multiply it by the depth of the coin. So times dx. And if I wanted to find the volume of this entire solid, this entire rotated solid, I would want to find the sum of all of these volumes. So this is just the disk right over here, but I could have another disk, a similar disk that I do right over here. I could have another disk right over here. And I want to take the sum of all of those disks.