Perfect

This is what we call a great mental effect!

Show two identical brass tubes. There is only one little difference. One of the tubes has a small ring-ornament on the top. Invite now one of your spectators to an experiment. The one tube with the ornament he holds in the left hand, in the right hand the other tube. The spectator is now asked to put a match into one of the tubes and close it again. He has to do that after you have left the room! You can even be accompanied by another spectator. So no chance for you to see anything!
As soon as the spectator has put the match inside the tube and closed it again you will be asked to enter the room again. There is no insider who could help neither anything else to cheat. But you will be able to name the correct tube as soon as you enter the room!

Difficulty: