Put a marble in the round depression and try to roll the marble into the pit in the middle.You can either hold it with your bare hands, or you can place it on a ping pong paddle. After printing it, I noticed that the starting round depression is too deep at 1 mm. If I improve this model, I'll replace the depression with half-circle wall about 2 mm tall. Because the starting depression is too deep, I place the marble right next to the depression to start. After about 100 tries I haven't rolled the ball in yet, but I've gotten about an inch away.