This has a large area of external convex surface with sharp edges all round. The reverse side has large tapering fracture surfaces, with a very small central area corresponding to the internal manufactured surface of the jar. The fragment falls onto whatever is immediately beneath the jar at the time. The jar, being empty, is thrown away without the consumer even being aware of the breakage. The fragment, when found, is then associated with whatever it fell onto, and the true origin is forgotten.