Be an Engineer According to Britannica, a "bridge is a structure that allows people and vehicles to cross over an open space. Bridges span, or stretch across, deep pits in the earth, bodies of water, and roads. The primary function of a bridge is to carry traffic loads: heavy trucks, cars, and trains." 1. Watch "What Makes Bridges So Strong" by SciShow Kids. 2. Create a drawing of a strong bridge and explain what makes your bridge strong.