The blackboard of the middle ages
A large steel shield in the form of a sea-shell attributed to Athena, the goddess of civilization, craft wisdom and war.
A Castle or better a fortified tower is floating over the country side and being attacked by goblins (or other monsters, depending on PC strength)
If thrown into a moat or a ditch, they react with water to make an area of ground a foot wide and solid enough to walk on rise from the bottom. If eaten-well, the body of the one who ate one of them is a real mess. Used to storm moats and forge rivers, streams and ditches. Must be kept dry until used.