According to Herodotus's Histories, Demaratus, then in exile at the Persian court, sent a warning to Sparta about Xerxes's pending invasion. In order to prevent the message from being intercepted by the Persians or their vassal states, the message was written on a wooden tablet and then covered with wax. "The Spartans", presumably the ephors, Gerousia or the kings, did not know what to do with the seemingly blank wax-tablet, until Queen Gorgo advised them to clear the wax off the tablet.