Sheep need a source of salt in addition to their grass and water.  The local geology doesn’t offer them much, so I provide them with a commercially-prepared mixture of sheep minerals — sodium chloride plus a bunch of trace elements — in a mineral feeder that keeps the salt from turning into a sodden mass when it rains.

The sheep will ignore the minerals for weeks at a time, and then suddenly decide that they have a desperate craving for salt.  I have no idea what drives the boom/bust cycle.

The most puzzling part of the whole endeavor, though, is how they manage the geometry of pooping in the feeder.