The demonstrator will be at the top of a ladder. They will have two bocce balls on a device that will fire one horizontally and the other will drop vertically at exactly the same time. A question will be asked as to which will hit the ground first. The audience will be asked to listen carefully for the sound of the bocce balls when they hit the floor. The object is to see whether they hit at the same time or not. Will there be one sound as they both hit the floor, or will there be two, one for each as they hit at different times?
How it happens
When the two are released, the fact that one ball is being propelled forward has nothing to do with the fact that it is being accelerated downward. The two directions are perpendicular to each other, and hence have no effect on each other. The two balls will hit the floor at exactly the same time.