Magic Is the Simplest Thing is the 20th chapter of Eragon.
As they ride, Brom teaches Eragon how to use magic. Apparently, magic takes up energy. All Riders could use magic. He also teaches him a few words in the Ancient Language. They track the Ra'zac up the Ninor River, while Saphira flies overhead. Brom also foreshadows The Word when he answers Eragon's question "Does this language have a name?" by saying "If there was such a word, it would be a word of great power."