BASE FIVE

An early, careful introduction to the idea that although base ten is ""the easiest way for us to count because we know it best, it is not the only way to count."" After reviewing how the familiar base ten system works (and at this level it's important to be sure that's straight), Adler introduces base five, (the system we might be using if man had only one hand, by way of the common tally marks (with every fifth mark crossing out the previous four), and takes kids through a number of exercises with checkers to demonstrate that 14 (base ten) and 24 (base five) really mean the same number, as do 16 (base ten) and 31 (base five). We'd rank this among the most successful of the Young Math books.