Allow your students to design and build their own bridge. Then see which of their models will support the most weight.
Materials needed - toothpicks, mini-marshmallows (gum drops are stronger but are more expensive and may be seasonal in your stores), and the handout of basic bridge styles in the student handout section below.
Divide the students into groups.
Give each group several toothpicks, mini-marshmallows, and the handout.
Students are to build the strongest bridge they can using only the toothpicks and marshmallows.
When all models are complete tell each group they are now bridge inspectors. They trade models with another group and complete a thorough inspection of the new model - provide feedback to the builders regarding the plus and minus features of their construction.
Now for the test - using books or other classroom materials see which bridge will support the most weight. Class discussion can focus on why each bridge was strong or not.