How to Draw a Perfect Circle Freehand

I came across Vasari's text below and figured I'd make a quick video showing you how to "surpass greatly all other painters". Well, not all the posts can be long and well thought out.

(link to video)

Giotto draws a perfect circle for the Pope, told by Vasari:

Pope Benedict sent one of his courtiers into Tuscany to see what sort of a man he was and what his works were like, for the Pope was planning to have some paintings made in S Peter's. This courtier, on his way to see Giotto and to find out what other masters of painting and mosaic there were in Florence, spoke with many masters in Sienna, and then, having received some drawings from them, he came to Florence. And one morning going into the workshop of Giotto, who was at his labours, he showed him the mind of the Pope, and at last asked him to give him a little drawing to send to his Holiness. Giotto, who was a man of courteous manners, immediately took a sheet of paper, and with a pen dipped in red, fixing his arm firmly against his side to make a compass of it, with a turn of his hand he made a circle so perfect that it was a marvel to see it Having done it, he turned smiling to the courtier and said, "Here is the drawing." But he, thinking he was being laughed at, asked, "Am I to have no other drawing than this?" "This is enough and too much," replied Giotto, "send it with the others and see if it will be understood." The messenger, seeing that he could get nothing else, departed ill pleased, not doubting that he had been made a fool of. However, sending the other drawings to the Pope with the names of those who had made them, he sent also Giotto's, relating how he had made the circle without moving his arm and without compasses, which when the Pope and many of his courtiers understood, they saw that Giotto must surpass greatly all the other painters of his time.