There was indeed a river, according to legend, one drop of which would rob a man of his memory. Many people assumed that this was the river Ankh, whose waters can be drunk or even cut up
and chewed. A drink from the Ankh would quite probably rob a man of his memory, or at least cause things to happen to him that he would on no account wish to recall.
Terry Pratchett, Soul Music (via discworldquotes)