A net is a 2D shape that can be folded up into a 3D solid. Every cardboard box has a net — that's how it was cut from a flat sheet before being folded and glued.
Familiar nets
- Cube — 11 different nets exist (cross, T, staircase…).
- Tetrahedron — 2 distinct nets, both made of 4 triangles.
- Cylinder — a rectangle plus two circles.
- Cone — a circle plus a circular sector (a pie slice).
Use a net to compute surface area: just add the areas of the 2D pieces. That's why the formula for cylinder surface area is 2πr² + 2πrh — two circles plus a rectangle.