A cross-section is the 2D shape you see when you slice a 3D object with a flat plane. Cut an apple, a salami or a tree trunk and the slice is a cross-section.
Spin the solid
Cube
faces
6
6
edges
12
12
vertices
8
8
V − E + F = 8 − 12 + 6 = 2
drag to rotate
Slicing a cone
- Slice parallel to the base → a circle.
- Slice at a slant → an ellipse.
- Slice parallel to the side → a parabola.
- Slice straight down through the apex → two intersecting lines.
The cross-sections of a cone are exactly the conic sections — that's literally where the name comes from.