Trick with the Liver.

And they exhibit a liver seemingly bearing an inscription in this manner. With the left hand he writes what he wishes, appending it to the question, and the letters are traced with gall juice and strong vinegar. Then taking up the liver, retaining it in the left hand, he makes some delay, and then it draws away the impression, and it is supposed to have, as it were, writing upon it.

Ref 2.40.1