A sharp, uncluttered exercise in mathematical logic, presented via examples that are as engaging as they are to the point. The authors use two kinds of castle drawbridge systems to illustrate differences between AND and OR patterns, and set a guard to drawing up truth tables that work out the conditions under which visitors can cross. The AND-OR concept is then applied to train switches (""you can build models of them using Popsicle sticks. . ."") and finally to ordinary statements. This is the sort of pared down intellectual activity that the Young Math series does best; Madden does his part with the usual jaunty clarity.