Long ago when Finn was young he went to study with an old Druid called Aengus, who had dedicated the last seven years of his life to fishing, and nothing but fishing. He was trying to catch the Salmon of Knowledge. Every year, the salmon of knowledge swam into the Otherworld, ate a berry from the tree of life there, and swam back to this world again. The berries from the tree of life gave him all the knowledge of the world, and it meant that whoever ate this salmon would then have access to all knowledge, of this world and the otherworld. After seven years crouched over his fishing pole, Aengus was crooked and gnarled by the time Finn came to him, and he was skin and bone, because he neglected to do anything that would take him away from his fishing. Finn offered to stay with him, and hunt and cook for him, in exchange for Aengus taking him on as his pupil.
Aengus got his strength back under Finn’s care, and he taught the boy the art of composing poetry, as they sat by the riverbank. At long last, Aengus was successful in his quest. He caught the wily salmon of knowledge! He gave it to Finn to cook, but warned him that he was not to take even the smallest taste of the fish: it was for Aengus alone. Now, as Finn was cooking the fish over the fire, a blister rose up on its skin, which he burst with his thumb. He burnt his thumb on the blister, and put it in his mouth to cool it down. When the fish was cooked, Aengus took one bite of it and knew that someone had tasted it before him: the knowledge was gone! He asked Finn if he had taken even a small bite, and Finn protested that he had not. At last, he told Aengus that he had burnt his thumb on the fish, and put it in his mouth to cool it. Aengus said Finn might as well have the rest of the fish, as he already had all the knowledge in the world. Finn said that he didn’t feel any different, so Aengus told him that he had to put his thumb in his mouth to access the knowledge he needed. Any time after that, when Finn needed to know something, or see the future, all he had to do was put his thumb between his teeth, and he would know the answer.