A curve is the longest distance between two points.
An angle is a triangle with only two sides.
A polygon is a dead parrot.
An average is a thing that hens lay their eggs on--for example, "My hens lay four eggs a week on the average."
A circle is a round straight line with a hole in the middle.
If one angle of a triangle is more than 90 degrees, the triangle is obscene.
A polygon with seven sides is a hooligan.
The circle passing through the vertices of a triangle is called the circumcised circle.
The circular points at infinity are called Isaac and Jacob.
The basic postulates of geometry are a pencil, a ruler, and a eraser.
Source: Desmond MacHale's Comic Sections, 1993, pp. 73-74