In this activity, students construct adding slide rules, scaled with linear calibrations like ordinary rulers. Students learn to move these scales relative to each other in ways that add and subtract distances, thus calculating sums and differences. This is Activity A1 in the "Far Out Math" educator's guide. Lessons within the guide include activities in which students measure, compare quantities as orders of magnitude, use scientific notation, and develop an understanding of exponents and logarithms using examples from NASA's GLAST mission. These are skills needed to understand the very large and very small quantities characteristic of astronomical observations. Note: In 2008, the GLAST mission was renamed Fermi, for the physicist Enrico Fermi.