Hoop Dreams

https://commons.wikimedia.org/wiki/File:WhiteHouse_viaFlickr_HulaHoops_8735987624_035318723f_o.jpg

A memorably phrased puzzle from The Graham Dial: “Consider a vertical girl whose waist is circular, not smooth, and temporarily at rest. Around the waist rotates a hula hoop of twice its diameter. Show that after one revolution of the hoop, the point originally in contact with the girl has traveled a distance equal to the perimeter of a square circumscribing the girl’s waist.”

Click for Answer