A cross-section is the intersection of a solid with a plane, which is a two-dimensional figure that extends forever in all directions. For example, some cheese is sold in cylindrical blocks. If the cheese is placed on one end and sliced vertically, the slice will reveal a rectangle, as shown. This rectangle is a cross-section of the cylinder.

These pieces of cheese are thin cylinders. They are like cross-sections, but they are three-dimensional. All the cuts were made parallel to one another. By looking at the slices, or by stacking them up, we can figure out that the original shape of the cheese was a cylinder.

What if another cheese plate contained slices whose radii got bigger to a maximum size and then got smaller again? The cheese was probably in the shape of a sphere. A sphere has circular cross-sections. The size of the circular cross-sections increases as they get closer to the center of the sphere, and then decreases past the center.