However, according to Herodotus, Histiaeus was unhappy having to stay in Susa, and made plans to return to his position as King of Miletus by instigating a revolt in Ionia. In 499 BC, he shaved the head of his most trusted slave, tattooed a message on his head, and then waited for his hair to grow back. The slave was then sent to Aristagoras, who was instructed to shave the slave's head again and read the message, which told him to revolt against the Persians.
Source: Wikipedia on Histiaeus