Introduction: Tie a Knot Without Letting Go of Either End of a String*
Here's a simple and fun trick to do with your friends. Sorry if it's a little hard to get at first. I didn't notice how dark it would be and I didn't have any brighter material to use. I may get pictures later and turn it into a full instructable.
*Be warned that there is actually a special trick to it and you do indeed let go of one end at one point though the point is to lead others to believe that you didn't.