6 × 3 = 18. A hexagonal prism has 18 edges. Image caption, The vertices of a prism are at each end of the 3D shape. The number of vertices is twice the number of vertices of an end face.

A polygon is a 2D shape with straight sides that meet at vertices. Polygons have three or more sides. Have a look at these three shapes. One of them is not a polygon. Can you work out which?