Suspension bridges and cable-stayed bridges both use cables, but they connect them to the towers in different ways — and that difference matters when weight is applied. You build scale models of each type from balsa wood and twine, then add weights to a hanging basket one at a time until something fails. When the results come in, the cable-stayed design holds nearly twice as much before it breaks.

Shape determines how a bridge handles force. You build three bridges from popsicle sticks, each with a different structural design, then place weights on each one and measure how much it bends using a micrometer. The amount of flex tells you which design resists force most effectively — and comparing those results shows which shape distributes weight best.

Bridges rely on basic geometric shapes to carry heavy loads, and not all shapes perform equally. You build two toothpick trusses — one from triangles, one from squares — then set a bucket on top and add scoops of gravel one at a time. The number of scoops each truss holds before it collapses reveals which shape makes the stronger frame.