The teacher is invited to take a turn crawling across. It's a tight fit, but most are willing to give it a try. The most technical part of the presentation is to teach the children the difference between tension and compression. In a truss, some members are in tension, while others are in compression. Chains are brought out to help illustrate the difference.
Chains are strong in tension, but have no strength in compression.
First, one of the compression members in the bridge is replaced with a chain, and the children see that this will not work. The bridge would collapse. Then a tension member is replaced with a chain, and they see that the bridge is still strong. The children are given another turn to cross the bridge, and are asked to feel the chain as they cross. This helps them get a feel for how putting their weight on the bridge causes a force in the members.