Harpagos, the guy whom Astyages forced to eat his own son as punishment for not properly arranging for Cyrus’s death, eventually decided to make Cyrus the instrument of his vengeance, like you do. Fortunately for Harpagos, Astyages was cruising for a bruising: despite being a Mede himself, the Medes in his empire felt downtrodden and oppressed. Harpagos capitalized on this by convincing various Mede mavens and thought leaders that Cyrus the Persian, despite being only half-Mede, would be a much better emperor for the Medes than his grandfather Astyages the Ethnicity Traitor.
Step two of Harpagos’s plan to put Cyrus on the throne was to inform Cyrus of his intentions. However Cyrus was out in the countryside and Harpagos couldn’t travel or risk sending a messenger to Cyrus, for fear of tipping off Astyages. Instead Harpagos killed a hare and sliced it open without otherwise cleaning it. He stuffed a message scroll into the hare’s belly, sewed it up, and wrapped it in a net.
“Take this net,” he told a servant. “Carry it into Persia and give it to Cyrus.”
“Won’t people be suspicious, me coming from your household with a message for Cyrus?” asked the servant.
“Ah, but that’s just it! You don’t have a message! You just have a hare!”
“Be sure to tell Cyrus about the message inside the hare. I don’t want him getting confused and thinking he should just cook and eat it.”
This then was accomplished, says Herodotus, and we shouldn’t question it. Cut to Cyrus reading Harpagos’s note.
“Son of Cambyses, blah blah blah, take vengeance on Astyages, blah blah blah, gods are on my side, blah blah blah,” read Cyrus. “Get the Persians to revolt, march on the Medes, and my mysterious benefactor will arrange for the Medes to defect from Astyages’s rule and come under my banner, that’s the gist of the plan.” He considered this. “Sounds legit! I should do what this letter I found inside the stomach of a dead hare tells me to do, most definitely!”