Page 4 of "The Decision" (Book 18). Ax finds it necessary to explain the concept of money to his hypothetical Andalite readers. He says "Money is sort of an abstract human concept. You give amounts of money to various people in society and they in turn give you useful items". Ax's need to explain implies that the Andalites don't use money in their economy. What do they use instead to keep track of the transactions. How does their economy work? How are they financing their war against the Yeerks?