What goes up must come down. What goes up must come down. How about this one? Now in physics, when we're trying to understand how things move, especially if they're extended objects like this, we need to pick one point that we use to understand what it's doing. And that point is called the center of mass. The center of mass of a symmetrical object like this is just going to be the geometric center. The center of mass is going to be between along this axis here and halfway down there. So instead of looking at the rails or looking at where the object touches the rails, I want you to concentrate on the center of mass. So focus on this point here, which lies on a line that goes through the center of mass. Look at what that point's doing and then now it'll make a bit more sense what's going on. Did you see it? Let me give you a bit of a hint. If I show you the track, you'll see that the rail gets further apart up here. Have another look. Again, concentrate on the right in the middle of the wheel and see what it's actually doing. Did you get it? Let me give you a hint. Is the middle of this object moving up or down? Have a really close look. Stick a ruler on the screen of your computer if you can do that without scratching.