Where Are The Matches?

Effect: Two empty matchboxes and one containing some matches are moved around on a table. The spectators are unable to identify which box contains the matches.
Secret: All three matchboxes are empty. Attached to your right arm and hidden by your sleeve is a matchbox containing some matches. Shake a box with your left hand it sounds empty. When you mix up the three visible boxes you can make any one appear to be the full one, simply by shaking it with your right hand. The spectators think it contains matches because they hear the sound from the hidden box.