This is a wonderful effect that can be used as an add on, or natural progression after doing Stratosphere. It is also a stand alone effect.
A beautiful cabinet is shown. Three doors can be shown empty. At that point a spectator is asked to freely select a ball. When the corresponding door is opened for that ball, everyone sees that a red arrow is pointing upwards at that ball. You can talk about the colors of the traffic light as you do this effect making it a safety lesson for the children in attendance.