Within these jars of glass, metal and nourishing cranial fluid resides the disembodied brain of a powerful sorcerer, arch-mage or necromancer. Now liberated of the constraints and distractions of corporeality, and severed from artificial bounds of morality, the brain may focus all its energies solely upon the greater Externalities.
Such jars may be found scattered across time and space, examining every facet of existence. If found, the brain within the jar may be persuaded to grant one small service to a subject displaying adequate intellectual rigor, or who is at least not boring: 1) accurately answer one question about a person, place, or thing; 2) provide a cryptic, prescient insight; 3) divulge an interesting location containing treasures, 4) grant one spell to a wizard, sorcerer or other arcane spellcaster. Immediately upon completing the bequest, the jar slowly fades away, as it transcends the 68th Parallel of Xuln (or some other equally nonsensical temporality).
If attacked, or acted upon with any form of malice, the brain will immediately translocate to one of its many inter-dimensional sanctuaries.
Edit: Optionally, the GM may decide that the brain is willing to trade for information, though it should be noted that the brains are quite knowledgeable on virtually all imaginable, and unimaginable, subjects. Tell the brain something it doesn’t already know, and it will grant one of the four pieces of information indicated above. Then it translocates.