Consider a wheel consisting of two concentric circles of different diameters (that is, a wheel within a wheel, or a wheel with external axes). Then if we roll this wheel between two straight lines (at the bottom of each of circle), we can see that the distance developed by a point of the circle with greater diameter is apparently the same as the distance developed by a point on the small circle. So the wheel should travel the same distance regardless of whether it is rolled from left to right on the top straight line or on the bottom one. This seems to imply that the two circumferences of different-sized circles are equal, which is impossible—this is the paradox.