(Mineral) Crail is a common kind of rock or gravel found widely across the British Isles. Each individual stone (due to an as yet undiscovered gravitational property) is charged with 'negative buoyancy'. This means that no matter how much crail you remove from the garden, more of it will rise to the surface. Crail is much employed by the Royal Navy for making the paperweights and ashtrays used inside submarines.
|source : The Meaning of Liff|