Consider a wheel of radius 1 rotating about its axis with a frequency of 1 turn per second. Now imagine that there is a second, smaller wheel, the center of which is attached to a point on the circumference of the first. This second wheel is rotating on its own axis at a frequency of 7 turns per second. Finally, imagine a third, yet smaller wheel, whose center is attached to a point on the circumference of the second. This third wheel is also rotating on its own axis but in the other direction to the other two and at a frequency of 15 turns per second. A pencil put on the circumference of this third wheel would trace out a closed pattern of loops with two-fold symmetry.