It does require holding one end with your teeth, and having a very agile tongue. You can improve the muscles in your tongue by standing in front of a mirror and making your tongue into a tube, turning it over at the tip, or making it roll in waves. (Easier to show than to put in words.) Drawing o’s on the inside of your front teeth with the tip of your tongue is a good exercise. Eventually you get enough control to grab the free end of the stem and roll it onto itself with a little push, and voilà, one knotted cherry stem.