The basis of ortho-rhodescauttering is electrostatic repulsion. We're going to model electrostatic repulsion with two balloons that have charge on. One is hanging from the ceiling and the other is going to be held in my hand. And what's going to happen is, as I approach, you're going to see a deflection. You're going to see that the balloon that's hanging from the ceiling moves away as the incoming charge approaches. And this models in a backwards way what happened with Rutherford. In Rutherford, it was the incoming particle that deflected. And in this case, so it's a little bit different, but it gives a sense of what comes in. You can actually have the students play around with different variables in terms of what's going to create the largest deflection from vertical. Does the length of the string matter? Does the amount of charge matter? Does the speed of the incoming particle matter? Let the students play with it and let them decide or discover for themselves what are the important variables in this really simple demonstration of electrostatic repulsion that is the basis of Rutherford scattering. Now, it's very important that the balloon that's hanging from the ceiling having a little bit of space in it. So, the students, if you're going to have a little bit of space in the ceiling, have a uniform charge all around its surface. If it doesn't have a uniform charge, then what's going to happen is when you approach it with a balloon that is charged, it's not going to swing away, it's going to spin away. And we don't want that. So, the balloon that's hanging has to have a charge all across all around it. It's going to be rubbed by something by wool or something to give a good static charge all the way around it.