Gurstelle also wrote "Building Bots: Designing and Building Warrior Robots", but I haven't read that one. "Catapult" is definitely in the spirit of "build it yourself", that I like to encourage here and also in "The Cost-Effective Teacher" feature in the print Journal. Gurstelle is an engineer, and he brings an engineer's perspective to the book. As in "Backyard Ballistics, he provides the colorful of the machines that he describes, but he also tells the reader how to tie the necessary knots and how to work with PVC pipe, which is used for some of them. Both of these skills can be useful in other contexts, of course. Projects like these have become very popular in physics classes, partly as a consequence of the excellent Nova program on ancient artillery. These would be excellent father-son-or-daughter activities but youngsters and (especially) exuberant teenagers need adult supervision when these things fire tennis balls or water balloons. They can cause damage or injury. [Isn't that part of the fun?] Chapter 1 is on safety, but cautions are also sprinkled as necessary through the descriptions of the construction and operation of ten different machine models. Part of Chapter 7 shows how to make your own rope, in a manner similar to that used by our clever ancestors.