Salmon of Knowledge
The Salmon of Knowledge (Irish: bradán feasa) is a creature figuring in the Fenian Cycle of Irish mythology. (It is sometimes identified with Fintan mac Bóchra, who was known as "The Wise" and was once transformed into a salmon.)
The Salmon story figures prominently in The Boyhood Deeds of Fionn, which recounts the early adventures of Fionn mac Cumhaill. According to the story, an ordinary salmon ate nine hazelnuts that fell into the Well of Wisdom (aka Tobar Segais) from nine hazel trees that surrounded the well. By this act, the salmon gained all the world's knowledge. The first person to eat of its flesh would in turn gain this knowledge.
The poet Finegas spent seven years fishing for this salmon. One day Finn Eces caught Fintan and gave the fish to Fionn, his servant and son of Cumhaill, with instructions not to eat it. Fionn cooked the salmon, turning it over and over, but when Fionn touched the fish with his thumb to see if it was cooked, he burnt his finger on a drop of hot cooking fish fat. Fionn sucked on his burned finger to ease the pain. Little did Fionn know that all of Fintan's wisdom had been concentrated into that one drop of fish fat. When he brought the cooked meal to Finn Eces, his master saw that the boy's eyes shone with a previously unseen wisdom. Finn Eces asked Fionn if he had eaten any of the salmon. Answering no, the boy explained what had happened. Finn Eces realized that Fionn had received the wisdom of the salmon, so gave him the rest of the fish to eat. Fionn ate the salmon and in so doing gained all the knowledge of the world. Throughout the rest of his life, Fionn could draw upon this knowledge merely by biting his thumb. The deep knowledge and wisdom gained from Fintan, the Salmon of Knowledge, allowed Fionn to become the leader of the Fianna, the famed heroes of Irish myth.
Stories differ on whether Fintan was a common fish or one of the Immortals, that could be eaten and yet continue to live.