Active Reading Solution: A Logic Gate

In order for current to flow from the "High V" lead to the "Low V" lead, it must flow through both transistors: first the top one, and then the bottom one. The book chose the convention that a high input voltage to a transistor allows current to flow, while a low voltage blocks current. (Remember that a transistor could easily be made the other way around.) Based on that convention... This is therefore a design for an "AND gate"; the output lead is high if, and only if, both input leads are high.