Fill a plastic, or rubber, tube with water. Hold the tube vertically up into the air and with the thumb of one hand plug up the bottom hole of the tube and place it next to your ear (hide the fact that you are plugging the hole).
Next, call a friend, or victim, and tell him that you are hearing something really strange through the tube and he should come over and listen in on it. When he gets close enough to your ear to listen turn the tube towards his ear and release your thumb.
