You show the audience a tennis ball. Consider up to three - and inside the ball there is light. Light moves!
To achieve this effect is very simple. About three meters from the ball should be light source, for example, a simple light bulb. And the ball is a round hole with a diameter of up to one centimeter. When you show the ball to the audience, then cover the hole with your finger. Counting to three, turn the bulb opening to the light and removing the finger, open it. Here the audience and the impression that the ball of light appeared. And that the light was moving, you just have to move the ball up-down and left-right, but not rotate it.