The Menger sponge

Drag n to cut the cross through one level after another; stop between two levels and the holes are only partly cut. Each level splits every cube into 27, takes out the 7 on the cross through the middle and keeps 20, so after n levels there are 20ⁿ little cubes holding (20/27)ⁿ of the volume. By the fifth level just over a fifth is left, though the last holes are too fine to see; keep going for ever and the volume goes to zero.