TeachEngineering is a National Science Foundation funded, searchable, web-based digital library of standards-based engineering curricula designed for students of a wide range of ages and abilities, from kindergarten through the end of high school. This particular hands-on engineering activity was designed for ninth graders and uses balloons to teach skills related to estimating buoyancy, as well as problem solving and basic physics. The activity spans eight 90-minute classes and gathers students into groups of three in order to capitalize on interactive learning. The lesson plan includes an excellent summary, lists of objectives and materials, and then offers step-by-step instructions that guide educators through the implementation of this exciting activity.