I've figured out how to use ModifiableContactPairs to make a sphere collide with the *inside* of another larger sphere. https://github.com/spiderworm/unity-modifiableContactPairs-experiments The smaller sphere is allowed to pass through the top of the larger sphere, where it then collides with the inside of the shape. I'm interested in hearing about how I might improve this. The most interesting bit of code is here: https://github.com/spiderworm/unity-modifiableContactPairs-experiments/blob/e415f05daf2d18810fd0020657e7ac8fb4ef8ed1/Assets/Sphere Dropping Into Sphere/SphereDroppingIntoSphere.cs#L25 My next goal is to get a small cylinder to collide with the inside of the sphere. If you have any thoughts on achieving that, I'd love to hear them.