Pappus's centroid theorem

Pappus's Centroid Theorem is a way of figuring out where the center is of a 3D figure made of flat faces. To understand it, it's helpful to imagine you have a bunch of flat, square pieces of paper. You arrange them so that all the edges touch, and stack them up to create a 3D shape. Now, Pappus's Centroid Theorem helps you figure out the center of your 3D shape. It works like this: you draw a line from each corner of all the flat faces to the opposite corner on other flat faces. When you are done with this, you will have drawn a lot of lines. Then, each line will cross two other lines, creating four small boxes. You find the center of each of these four boxes, and draw lines through each one. The place where all four of your lines cross is the center of your 3D figure!