Today, Sam taught the kids about structural engineering and what it took to build a sound building. He began by illustrating tension and compression by bending a piece of sponge and showing that the top experiences tension and the bottom compression. He then explain how a building would fall if the center of mass of the building was not within the base. This was illustrated by a computer applet on Shodor's site that calculated where the center of gravities of structures were. Finally, the kids had a building competition on the computer. They were given a limit of how much material they could use and were asked to build a structure that could hold up fifty pounds of weight the best. This was measured by how much force was exerted in each member of the structure, the lower the better.