Let's create a sphere whose position in the 'y' direction is fixed to 0.0. In other words, the sphere is not capable of moving in the up/down direction.
1. Create an analytic sphere.
2. Select the sphere, open the property window and go to the Script tab.
center=Self.GetCenter(); center.y = 0; Self.SetCenter(center);
Now, try to move the sphere with the mouse. You can move it horizontally but not up or down!
Let's create a simple animation. Try the following script:
center=Self.GetCenter(); center.y = Time; Self.SetCenter(center);
On other words, the 'y' position of the sphere depends on the time. Play the animation to verify this.