Inquiry labs in the science classroom help students make connections and discover scientific relationships for themselves. This article describes an inquiry laboratory in which students explore the properties of a pendulum. Since most students are familiar with pendulums before they get to high school, they have discovered that the period is largely independent of the mass of the pendulum bob, but sensitive to differences in string length. Many students may even understand that as a pendulum's amplitude decreases due to friction, it keeps roughly the same time; hence its usefulness for mechanical clocks. To engage students in learning more about the properties of a pendulum, the author suggests that science teachers can ask students to discover and explain a relationship between the length of time for a swing and the string length. Teachers might think that such geometric reasoning is beyond high school students, but that is the special value of this exercise: it is remarkably approachable. Making an approximation, shifting from an arc to a straight line, and applying some geometry simplifies the problem to the point where most students can discover the essential relationships and apply the geometric analysis. (Contains 2 figures.)