"This is another CLASSIC mini illusion. It's super easy to do (natch!), and looks like something supernatural. The gimmick is beautifully made, and nobody will ever find the secret. Looking for something to demonstrate that you are an amazing magician. You just found it! "
You openly show the brass tube and the steel ball, and the ball is clearly too large to fit in the tube. However, on your command the ball slowly sinks down into the tube. Now on your command the ball slowly rises back to the top of the tube. Here's the cool bit - you now immediately hand out the props for examination. They will find nothing, and they will not be able to get the ball to go into the tube. It just won't fit. No! There is no switching! Again, everything is examinable.