Lay a short rope on the table and form a figure eight with it.
Take the longer end of the ropes and make a circle around the eight with it. The ends of the rope should both be coming toward you.
Have a volunteer put his finger in the center of the top of the eight while you pull the ends of the rope.
Notice that his finger has two loops tightened around it now.
Put your own finger in the same loop, and the rope comes free ' no loops for you!
The trick is in the way the rope is laid out.
Make the figure eight so that the short end of the rope ends up on the left. This means that the long end will cross the short end as it completes the eight. The long end is then taken to the left and around the top of the eight, ending up coming back down to lie parallel to the short end.
Lay the short end over the rope if you want to trap the finger.
Put the short end so it crosses under the rope if you want it to pull free.