Knot from nowhere

effect: a knot mysteriosly appears in the corner of a handkerchief
secret: before your performance,secretly tie a knot in one corner of a handkerchief. Show the handkerchief but hold it with the knot hidden in your right hand.
The left hand now lifts the bottom end of the handkerchief and places it in the right hand a long with the top corner. Give the handkerchief a shake and drop the unknotted end. Repeat this two or three times. On the second or third attempt allow knotted corner a to drop. Retaining your grip on the other corner. All you have done is change the corner you are holding but it appears that the knot has formed by magic