Trick Effect & Routine:
A magician throws a piece of rope in the air. When it lands on the floor, a knot has been tied in it!

Preparation:
You need a piece of rope, suprisingly enough.

Method:
Hold your right (or left hand, it makes no difference) flat out, palm facing the ceiling. Put a length of rope between your pinky and 3rd finger, and clip it there. The other end of rope should be draped across the hand, between the thumb and the side of your hand. The rope hanging here shouldn't be any more than a foot long. Turn your hand over, and clip the piece of rope held between the thumb and side of hand with your index and middle fingers. Throw the rope in the air, letting the loop you've created slide off your hand, but keep the piece of rope you've clipped between your index and middle finger clipped until the very last moment, when you let it go. There should now be a genuine knot in the rope.