The "snailball" is a magic trick wherein an apparently solid metal sphere rolls slowly and erratically down an inclined plane. The key to the trick is that the sphere is not actually solid throughout, but a shell containing a second, smaller sphere with a very viscous fluid sandwiched between. As will be demonstrated in the seminar, the same phenomenon occurs when a combination of cylinders rolls down a surface. A mathematical model of the latter is presented to explore in more detail the motions of the two cylinders. A key feature of the predicted dynamics is that the inner cylinder cannot be suspended in the fluid by the rolling motion of the outer cylinder. Instead, it must always fall toward the outer cylinder and "contact". The rocking and rolling motion of the real cylinders must then incorporate a key extra physical detail, namely what happens when the cylinder surfaces become very close. Two possibilities are discussed: contact between rough surface, and cavitation.