One small sewing needle or pin.
Hold the pin up in the air and tell the participants that you are going to conduct an experiment. You are going to drop the pin on the desk and would like everyone to be quiet so they can all hear it. Drop the pin and have all participants who heard it raise their hands. Repeat the experiment until each has a chance to hear the pin drop.
This experiment can be compared to how one can hear the whisperings of the still small voice. Ask why the Spirit is often described in this way. Why do they think it is also described as being something that pierces to the center and causes your bones to quake? Explain that the Spirit whispers so that only those who are listening can hear it. Do your daily surroundings make it easier or more difficult to hear the Spirit of the Lord?